Commits 1st implementation of new Stepp method

This is the first new implementation of the Stepp completeness method. It includes the completeness directory, base class, Stepp (1972) class and changes to the hmtk.seismicity.utils.py function. All methods have test coverage; however, some tests are circular for the time being. The completeness function is tested using the completeness_test_cat.csv catalogue and is proving acceptable (compared to other methods). It will be necessary to expand the test coverage with further tests in the future.