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.
Merge pull request #3094 from pllim/analytic-func

Analytic functions with blackbody