Resolves #289

• Refactored backend module to address entrypoint not found
• Refactored compat module to handle both packages and subpackages
• Made statsmodels and pyscagnostics optional
• Added importorskip for unit tests so that tests will skip when dependency is not installed. This was applied to some units tests.
• Added new pytest mark called "base" which identifies tests for only base data-describe i.e. without optional dependencies
• Updated dependencies to minimum working versions, in conda yaml files and setup.py
• Added developer guide to docs