Allow the user to specify the number of fragments in a library.

[emarinri]

Is your feature request related to a problem? Please describe.
It would be nice (although perhaps not necessary) to allow the user to set the number of fragment configurations in the library.

Describe the solution you'd like
Add the number of fragments in the library to mosdef_cassandra.print_valid_kwargs().

Describe alternatives you've considered
None

Additional context
This feature is probably not necessary but it would be an improvement.

For rigid molecules only 1 configuration is really needed, so large files that are largely unused can be avoided.

Note that a valid simulation can be conducted with a library of many conformations for a rigid molecule by generating an initial configuration of the rigid molecules and using only translation and/or rotation moves.

For a mixture of flexible and rigid molecules, the user can set the percent of the attempted regrowth moves of the rigid molecule to zero, which will prevent Cassandra to use a library of fragments for the rigid molecule.