Implement a Milky Way density model

[hombit]

Feature request

I've implemented a couple of simple Milky Way stellar density models here:
https://github.com/uiucsn/ztf-dr-vars/blob/master/ch_vars/spatial_distr.py

I propose to port them to tdastro, starting with MilkyWayDensityJuric2008 as the simplest one

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• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
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