Need to validate modifications to the subpop* functions

[LimaRAF]

@gdauby

I will push soon to devel the new function 'subpop.radius' and all the edits and additions I did in 'subpop.comp' and 'subpop.estimation'.

Everything I done are suggestion and it would be nice if you could take a look at them. I left several notes along the code so that you can trace back were I made changes and points that need some thinking before we do any fursther changes.

Some details of these modifications:

• subpop.radius: function used the package 'data.table'. Not sure if other functions use it, but it may be the case to code it without it if we do not want to inflate too much the number of package depencies.
• subpop.comp: it now accepts species-specific radius. I also added the argument 'export_shp', which is transmitted to 'subpop.estimation', so that the user can define it he needs the 'sf' objects to be returned. Therefore, I neede to add some if/elses along the way. In addition, I do not provide any default since the head of SPV said "For these types of settings (incl. those related to location, severe fragmentation, etc.), I think providing default values should be avoided, as they tend to restrict the user's thinking."
• subpop.estimation: added the argument 'export_shp' and adapted the function to handle and return only 'sf' objects. I also make the union of overlapping circles, to get the unique subpopulations. Not sure if this was done before (needs checking).

Let me know if you have any questions on my last commit regarding those changes.