Allow configuring of initial point for pymc models

[jessegrabowski]

So far I'm just configuring and returning the pymc initialization function. My goal has been to re-use as much PyMC code as possible.

Remaining issues:

• Fix return type of the initial point function (currently it's a dict of arrays, what does it need to be?)
• Expose initialization arguments (default_strategy, jitter_rvs, overrides) somewhere. In compile_pymc_model I guess? This will change the API somewhat -- currently init_mean (which does nothing) is on sample.
• Add rust function to use the provided python function

[aseyboldt]

Thanks!
Looks good to me. I changed to default to support_point after all, models with improper priors won't work otherwise.
I'll merge if the tests pass.