Separate header and extend collections.

[SGSSGene]

sargparse implements an incomplete is_collection trait.
Currently it is only applied to std::vector and std::set.

I suggest to extend this to more types like std::list, std::unordered_set and maybe more?!

Also I would suggest to introduce a header for each trait. Otherwise it is necessary to include all the StandardLib headers my default which is not wanted.

Second this could also be used to introduce a general "sargparse.h" header.

Code could look than like this:

#include <sargparse/sargparse.h>
#include <sargparse/vector.h>