Handle integer input into cosmology methods.  Note where 1D arrays expected

* Closes #1815
* Removes loop in nu_relative_density thanks to suggestion from mhvk.
  As a result, nu_relative_density now accepts arbitrary dimension arrays
  of redshifts.
* Note in documentation where 1D arrays are expected.
* Some other changes to enforce floating point return.
* Add unit tests for integer inputs in test_cosmology.