Fix for #1900
Don't run tests if scipy is not present
Merge pull request #1903 from nden/linear-models-deriv

Fix for #1900
Update CHANGES.rst

Changelog entry for #1900.
Update CHANGES.rst

Ugh, a3616a8089bea8ad05f987f4ae8e7569fc746045 should have referenced #1903.
Bug fix for attribute lattyp

The code on lines 814 and 784 had attribute self.wcs.lattype which raised an AttributeError. The class in question has attribute

astropy.wcs.Wcsprm.lattyp

So I edited those two to reflect the name
added test for the attribute error I got prior