Add function to calculate inverse of cosmology function and tests
Add changelog entry
updates based on feedback
minor doc updates
add description of z_at_value along with some other minor clean-ups
Fix test failures introduced by doc changes
more changes to address travis build failures
further changes based on feedback from @eteq
fix a couple of typos
Adjust round-trip z_at_value tests to exclude unsuitable functions, instead of listing everything that needs testing
Merge pull request #1909 from nhmc/add_cosmology_inverse_function

Add an 'inverse' cosmology function