assertWcsNearlyEqualOverBBox and friends is too hard to use as a free function

assertWcsNearlyEqualOverBBox and similar functions elsewhere in afw were written to be methods of lsst.utils.tests.TestCase, so their first argument is a testCase. This is fine for use in unit tests, but a hassle to use as free functions because the user must provide a testCase argument (though it need only be a trivial class with a fail(self, msgStr) method). Worse, that minimal requirement is not documented, so technically providing a simple mock test case is unsafe.

I have two proposals:

• Document the fact that testCase need only support fail(self, msgStr). This makes it clear how to safely use these functions as free functions.
• Allow testCase to be None, in which case RuntimeError is raised. That makes these functions even easier to use as free functions.