Create method for CLVModel to attach str_for_dist to parameters

[larryshamalama]

Priors are initialized via the .dist() API and are manually attached in the _process_priors method.

pymc-marketing/pymmmc/clv/models/gamma_gamma.py

Lines 33 to 36 in d91ba13

	# Who had the idea of attaching methods to pre-existing objects?
	p_prior.str_repr = types.MethodType(str_for_dist, p_prior)
	q_prior.str_repr = types.MethodType(str_for_dist, q_prior)
	v_prior.str_repr = types.MethodType(str_for_dist, v_prior)

This is currently true for the Gamma-Gamma models and BetaGeoModel in #67. A helper method in CLVModel could do the trick. An even better solution would be to address issue 6311 in PyMC.