Better check the species x category object

[ahasverus]

It could be great to check if all species present in the traits x species object are missing in the species x category object. This check can be add in the function check_species_categories() or in different function.

Then we can choose to return an error or to print a message to inform the user about this potential issue.

Note that we don't do this for the traits list, so maybe it's useless.