Add comparative tests with master branch on examples

TODO:

• add a script in order to install airGR package from cran, browse manual files and for each run the example and record all the variables created by each example. This record will be a file saved in artefact for use in next CI jobs. This script should be notices in .Rbuildignore.
• Modify .gitlab-ci.yml this script should be run before build and test of the package.
• Add test that run the examples and compare the variables with the one coming from the example executed from the CRAN version package.