Trigger theano model compilation at install time

[bsipocz]

It can be a rather disheartening first time experience to wait minutes for the theano compilation. Would be much nicer to do it at install time for all astroML implemented models.

[bsipocz]

The compiled models are usually cached in the user's home under .theano. Another task is to see how could files propagated to users if the package is centrally installed (and trigged compilation).

[bsipocz]

Another issue: if pymc3 is an optional dependency, we certainly cannot provide this functionality when it's not installed. But that's OK I think, those users will see the first time running the pymc3 reliant code to be slightly slower.