Add table of built-in cosmologies, other minor documentation cleanups.

* closes #1147
Fix sphinx warning, add note about doc update

* Extra space before code block in Built-in Cosmologies section
  of index.rst
* Add warning to parameters.py to update the documentation table
  when adding a new cosmology
Allow tuples to be passed to is_equivalent
Added note to changelog
Added a few negative assertions to test
Don't need to use an explicit list
Make sure the list of modules being iterated over cannot change during the iteration.
Merge pull request #1527 from aconley/cosmo_builtin_docs

Add table of built-in cosmologies, other minor documentation cleanups.
Merge pull request #1533 from astrofrog/fix-catch-warning-issue

Make sure the list of modules being iterated over cannot change during iteration
Merge pull request #1521 from astrofrog/unit/is-equivalent-tuple

Feature request: have Unit.is_equivalent take tuples