parameters do not have consistent types

[MarcAntoineSchmidtQC]

Parameters p (power in the Tweedie distribution) and theta (from #627), are handled in an unintuitive way. They request that the input be a 64-bit float or integer, then transform this into a 32-bit float. This means that setting the value of these attributes to the same value it currently holds raises an error. Example:

dist = glum.NegativeBinomialDistribution()
dist.theta = dist.theta  # This fails.