Add tests for email module

[ItIsJordan]

Currently email functions are not directly tested, and is also excluded from coverage as noted in .coveragerc. Functionality is mostly (assumed) tested indirectly through other tests, and through the E2E testing process. This should be improved by directly testing the functions contained in the email module in their own file: email_test.py.

Task list:

1. Add tests for email module into tests/email_test.py
2. Remove */hepdata/modules/email/* omit from .coveragerc
3. Ensure coverage decrease does not occur