DOC: DocExample default algortihm is now som instead of pca (compilation issue when building without shark)

This was done with this script:

    #!/bin/bash

    set -eou pipefail

    # Use C++11 sstream constructor from string
    # Exclude 'ThirdParty' because it uses ossimFilename type
    find Modules/ Examples/ -not -iwholename "*ThirdParty*" -type f -print0 | xargs -0 sed -i -E "s/std::ifstream (.*)\((.*)\.c_str\(\)/std::ifstream \1\(\2/"
    find Modules/ Examples/ -not -iwholename "*ThirdParty*" -type f -print0 | xargs -0 sed -i -E "s/std::ofstream (.*)\((.*)\.c_str\(\)/std::ofstream \1\(\2/"

    # Use C++11 ::open(const std::string&) of ifstream, ofstream, etc.
    # Exclude ThirdParty because of ossimFilename type
    find Modules/ Examples/ -not -iwholename "*ThirdParty*" -type f -print0 | xargs -0 sed -i -E "s/\.open\((.*).c_str\(\)/\.open\(\1/"

    # No need for c_str when outputing to std::cout and such
    find Modules/ Examples/ -type f -print0 | xargs -0 sed -i -E "s/\.c_str\(\) ?<</ <</"

    # Use itk::ExceptionObject::SetDescription and SetLocation string versions
    find Modules/ Examples/ -type f -print0 | xargs -0 sed -i -E "s/e\.SetDescription\((.*)\.c_str\(\)\)/e\.SetDescription\(\1\)/"
    find Modules/ Examples/ -type f -print0 | xargs -0 sed -i -E "s/e\.SetLocation\((.*)\.c_str\(\)\)/e\.SetLocation\(\1\)/"

    # Use itksys::SystemTool:: string versions
    find Modules/ Examples/ -type f -print0 | xargs -0 sed -i -E "s/itksys::SystemTools::(.*)\((.*)\.c_str\(\)/itksys::SystemTools::\1\(\2/"
    # Exclude otbLogger because it uses GetCurrentDateTime which does not have a string version
    git checkout --  Modules/Core/Common/src/otbLogger.cxx
    # Exclude test/otbOGRLayerStreamStitchingFilter.cxx because there's a double use which is not compatible with the regex
    git checkout -- Modules/Segmentation/OGRProcessing/test/otbOGRLayerStreamStitchingFilter.cxx

Before this commit, many files are using std::string without including
<string>. It can work accidentally but causes issues when refactoring,
especially if using operator <<() which is included implicitly by some
compilers.

To find guilty header files, I used:

    grep -l "^ *std::string" $(grep -L "#include <string>" $(find . -type f -name "*.h"))

which finds all files containing "std::string" at the beginning of a line
(usually a member or variable declaration), but not "#include <string>".
And then this script to add the includes (plus some manual ediing):

    #!/usr/bin/env python3

    import re
    import argparse

    def fix_file(filename, header):

        with open(filename, "r") as f:
            content = f.read()

        matches = list(re.finditer(r"(#include .*\n)\n", content))
        if len(matches) == 0:
            print("no include!")
            sys.exit(-1)

        pos = matches[-1].end(1)
        open(filename, "w").write(content[:pos] + "#include <{}>\n".format(header) + content[pos:])

    if __name__ == "__main__":
        parser = argparse.ArgumentParser()
        parser.add_argument('--header', type=str, required=True)
        parser.add_argument('files', type=str, nargs='+')
        args = parser.parse_args()

        for filename in args.files:
            fix_file(filename, args.header)

OTB followed since the beginning the ITK convention and use .txx extension for all
template classes. Nevertheless, some development tools do not recognize .txx
file extension. Other tool like GitHub can't do in-browser syntax highlighting for
txx files I think.

The root problem is the use of the txx which should be changed to hxx (or hpp).

In 2011, after an in-depth discussion near April 20, 2011 on the
Insight-Developers mailing list, ITK rename all txx files to hxx (and event
prevent the push of .txx files with a pre-commit hook). It happens is major release v4.

You can find some arguments in the discussion about the change and also in other
projects related to ITK which applied the same modification, see for instance VXL:

https://github.com/vxl/vxl/issues/209

This commit apply now the same modification for OTB.

I understand that it will change some habit for developers and don't bring new
features but I think that in general it is better to stay align with ITK guidelines.

In my opinion, it always facilitate the use of OTB and ITK together if we share
when we can the same code architecture, directory organization, naming
conventions...

STY: Change ITK_OVERRIDE for override in whole code (find Modules -type f -exec sed -i 's/ITK_OVERRIDE/override/g' {} +)