New load_likelihood_from_module

[vitenti]

This PR introduces a new loading method to the system, which permits the direct loading of likelihoods from individual modules. This enables multiple likelihoods to be organized within a single Python package.

[vitenti]

@tilmantroester does this PR address #340 ?

[tilmantroester]

Yes, I think so.

Being able to specify the entry point (i.e., a function that's not necessarily named build_likelihood) would be a useful feature, so that different likelihood factories can be defined in the same file/module. But that's probably best done in a separate PR.