Improve object/class names

[davharris]

• [x] priors should probably be weight_priors or something, since priors can go elsewhere
• [x] error_distributions should probably just be distributions, since they can also refer to priors
• [x] networks should probably be mistnets, in case I want other kinds of networks (e.g. with other kinds of latent variables)
• [x] par_skeleton might be better as adjustable_parameters or just parameters, given the way it's used.
• [x] make_gamlss_distribution should probably just be make_distribution

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