Numerical start and stop force unit='year'

[cliffckerr]

Numerical start and stop force unit='year', e.g. the first one only has 21 timesteps, while the second has 241:

import starsim as ss

kw = dict(diseases='sis', networks='random')

s1 = ss.Sim(unit='month', dt=1.0, start=1980, stop=2000, **kw).run()
s2 = ss.Sim(unit='month', dt=1.0, start='1980-01-01', stop='2000-01-01', **kw).run()

Originally posted by @cliffckerr in #798 (comment)