Convert to storing ell_min,ell_max instead of LM data

I want to avoid situations where people try to store (2,2) data only,
for example.  So I've switched from keeping the LM data as a primary
part of data to storing ell_min,ell_max fields.  In fact, they are
stored as __ell_min,__ell_max, but can be retrieved through a property
and set through a setter.  LM is now a read-only property, referring
to an array that is stored "privately" in the class, but is recomputed
each time ell_min and ell_max are set.