Move method-level import in GenericMethodWrapper.__init__() to module-level

[yousefmoazzam]

Very similar to #450 (in terms of the issue, and in terms of being needed for an integration test to go into #446), see that issue's description for details.

And similarly, any tests that attempt to patch import_module in generic.py via the following:

httomo/tests/method_wrappers/test_generic.py

Line 63 in 5c5c57b

mocker.patch("importlib.import_module", return_value=FakeModule)

need to be changed to patch the import like so:

importmock = mocker.patch("httomo.method_wrappers.generic.import_module", return_value=FakeModule)