add some tests
2to3
clean up tests
Simplify visible NM calculation

Should match ancient method more, elim 31-day months

fix limit in metonic fn
bab tests: to fixup
Use astronomical notation for years BCE
simplify setting epoch

clean up ruler epoch getting/setting
Add babylonian to init
moving to 687-year super-metonic cycle
better tests
make metonic monthlist builder 22-year aware

break up subfunctions
adjust moon visibility functions to remove gotchas