Add spin from pulsar frequency to conversions

[cdcapano]

Adds a function to compute dimensionless spin given a pulsar's mass, radius, and rotation frequency. Just assumes the pulsar is a solid sphere when doing so. Example:

>>> from pycbc.conversions import spin_from_pulsar_freq
>>> spin_from_pulsar_freq(1.44, 11., 500.)
0.3975488979662235

[spxiwh]

This looks good, other than things codeclimate has already mentioned: Don't use "I" as a variable name. mt also doesn't pass PEP-8 conventions for variable names (at least 3 characters long and isn't really self-descriptive ... But we can find plenty of such examples within PyCBC code ).

Just need to make sure that any users are aware that this will just be an illustrative measure of the moment of inertia (because of the assumptions clearly stated in the help text). I can't think of any use case where that is a problem though!