Masking a Mixture Model makes `component_` methods unavailable

[nstarman]

The MaskedDistribution object wraps models, but does not necessarily expose their public API. I'm raising this Issue to perhaps add support for the components_ methods.

Possible Options:

1. Some clever method of auto-wrapping methods accessed by __getattr__.
2. Have a MaskedMixtureDistribution class that is returned by MixtureDistribution.mask().
3. instead of .mask() masking the Distribution, it returns the same mixture, but calling .mask() on all the component distributions.