korad-kaxxxxp: factor voltage/current capability out of models[] table

korad-kaxxxxp: factor voltage/current capability out of models[] table

Move the number literals for supported voltage and current ranges and
their granularity out of the models[] table. Reference the ranges from
the models[] table by symbolic names, such that entries will read as
"a model which supports up to 30V and 5A", which is considered easier
to maintain. It's assumed that only few of these "templates" will be
required, the current set of supported devices gets away with 30V/60V,
and 3A/5A, and is able to express all of their combinations. See the
Korad KD6005, Tenma 72-2535, and Tenma 72-2550 models, which are the
only exceptions from the typical 30V/5A configuration.