Use unit support in Astropy models

[pllim]

Now that astropy/astropy#4855 is merged, we can refactor the code to use that feature. But that feature is only for Astropy v2 or greater.

Related:

• Enable unit support on all functional models astropy/astropy#6183
• https://gist.github.com/pllim/babea1d1337da665740439887c5b6f5f
• http://astropy.readthedocs.io/en/latest/modeling/units.html

[pllim]

I thought about making this change backward-compatible but it is not feasible. Not only it reduces performance but also increases code complexity. Since this package is still in "beta" version, I think it is okay to require Astropy version to be 2.0 or greater without much consequence.

c/c @bhilbert4

[pllim]

This now needs Astropy v3 to work due to astropy/astropy#6529

[pllim]

Let's wait for Perry's astropy.modeling refactor. This package will still work as-is for now, by passing unitless values to underlying models and wrap unit handling around those.