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

Before submitting Please check the following:

• [ ] I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• [ ] 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.
• [ ] If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.