Justify default values for lo and hi

[jeromekelleher]

Not obvious where these came from. In particular, why 2 and not 1 (or zero) for lo?

I've put a placeholder into the MicrosatMutationModel class.

[andrewkern]

these defaults weren't chosen for any good reason. lo=2 makes sense as a default rather than 1 because these are repeats-- so repeat number > 1 by definition (usually)

[jeromekelleher]

Sound reasonable to me - we just need to document these choices. People will assume that the defaults are the Right Choice.

[andrewkern]

going to close this out now that #2080

[jeromekelleher]

See #2086 for discussion on changing lo to 1.