Correction of minor bugs, documentation improved

Correction of bug for second maxima selection in coregistration ; minor bugs corrections ; begin to check compliance with lidR 3.0

Improvements in aba files

rastZonalStats function added, code removed from segmentation, possibility to perform detection and segmentation on dsm instead of chm. Still one bug to correct.

updated doc of ABA functions

treeExtraction output is a spatial data.frame
updated raster support inside treeSegmentation

- raster objects accepted as inputs of image processing functions
- minor modifications

added metrics.R