Maps each supplied species name to a conservative upper bound on the number
of days from conception to birth, using the speciesGestation
lookup table (or a supplied gestationTable). Matching is case- and
whitespace-insensitive. Any species that is missing, NA, an empty
string, or not present in the table falls back to default (210 days,
the conservative rhesus bound). Used by getPotentialParents to
key its gestation window on the first-class species pedigree column.