Improve the coverage of the astropy.cosmology unit tests

* Use the coverall output to add some unit tests for interesting,
  untested functions and branches.
* Add a Tnu method like Tcmb.
* Add a few missing np.exp (replacing exp) for array arguments.
* Closes #1653
Add a few more tests suggested by @astrofrog

* Test H0, K as quantities.
* Test FLRW.de_density scale directly.
* Test FlatwCDM
Add explicit tests of efunc equalling inv_efunc for all built-in de models.

* Add test method that compares efunc to inv_efunc for each class
  (LambdaCDM, w0waCDM, etc.)
* Fix doc string for FLRW about neutrino masses
* Fix missing 0 on ._Ode0 in wCDM
Mark test of FLRW efunc/inv_efunc as requiring scipy.

* Split off that test into marked function, keep other cosmologies
  in previous one because they shouldn't require scipy.
Merge pull request #1654 from aconley/cosmology_coverage

Improve the coverage of the astropy.cosmology unit tests