Enable coverage for Cython code

[adrn]

It seems like Cython supports Coverage analysis now?
http://docs.cython.org/src/tutorial/profiling_tutorial.html#enabling-coverage-analysis

I'm not very familiar with coverage, but to me it doesn't look like the Cython code in Astropy is included in the coverage report.

[adrn]

See also: https://github.com/snorfalorpagus/hello_cython/blob/master/setup.py

[nstarman]

I believe codecov, the current system, does this already.

[saimn]

We have coverage for C code but not for Cython. I did it for astroscrappy (astropy/astroscrappy#49), I guess something similar could be used for astropy.