Tentative fix for #2262.

__new__() and __setitem__() in Latitude (resp. Longitude) now check that
the input argument is not a Longitude (resp. Latitude), in which case
a TypeError is raised.
Added a couple more tests for the fix for #2262.
Updated the Changelog with info about changes introduced by #2461.
Merge pull request #2461 from bluescarni/bugfix-for-issue-2262

Tentative fix for #2262.