Let Quantity treat input with unit attributes more carefully.
Add test that equivalencies set in the context are respected
Add test for TypeError on encountering bad units; clarify exception text
Merge pull request #3051 from mhvk/quantity-input-with-unit-copy-is-false

Let Quantity treat input with unit attributes more carefully.
Fix #3108: Allow comparison of units and non-convertible-to-units
Add CHANGES entry
Merge pull request #3108 from mdboom/unit/compare-with-non-unit

Bug: unit comparison with None raises TypeError
added planck function to functional_models
update to planck function
added planck and test_planck() functions
back to baseline prior to moving planck stuff to new subpackage
Added new subpackage. Added Planck function by @skendrew from #1480 and related comments in that PR.
Modified planck function based on the one in pysynphot. Added tests and documentation.
Applied all review comments, updated tests and docs.
Improved check for unit attribute and removed xfail test, as suggested by Marten v. K.