species: don't warn for harmless duplicates

We used to warn like this:

  warning: "foo" has already been used to name a different taxon

even if in fact it had been applied to the /same/ taxon. Fixed.
species: improved genus handling

Better support for genera in the list of accepted species: they can be
more than just "Salix sp": have common names and aliases.  This is
mostly so I can use the name "majgr�s" for Hierochlo� spp.

A handful of such synonyms added.

Added documentation (groblad_species(5)) for the file format, and
some unit tests.
groblad 3.6