[improvement] Make lower.taxa function work with species names

[jaum20]

At current state, lower.taxa function works only with family and genus names. It will be useful to get infraspecific taxa, too. For instance:

flora::lower.taxa('Micropholis guyanensis')

Returns nothing character(0). But flora::lower.taxa('Micropholis') returns subspecies too:

(...) "Micropholis guyanensis subsp. duckeana", "Micropholis guyanensis subsp. guyanensis" (...)