Femethods

- Move common/private functionality into core module
- Add some type hints for mypy
- Restructure tests to use pytest fixtures and parametrized tests
- Add make commands for building and publishing to PyPi

- Update `Beam.plot` method to allow customization of plots generated
- Added tests for `Beam.plot`
- Added depreciation warning to `Beam.bending_stress` method

Backwards Incompatible Changes
- Removed `bending_stress` parameter from `Beam.plot` method

- Add documentation on [Read The Docs](https://femethods.readthedocs.io/en/latest/index.html)
- Expand module and function documentation
- Rename module from FEMethods to FEmethods (lower case m)

Backwards Incompatible Changes
- `Beam.plot` now returns a tuple to give access to the axes directly instead of just the figure
- `Beam.plot` defaults changed to not automatically plot the beam stress

- Add CI, coverage reports and expand unit tests
- Rewrite existing tests using pytest
- Invalidate `reactions` when when `elements.Beam` is invalidated
- Add warnings when differentiating near beam ends
- Add show method to wrap `matplotlib.pyplot.show` function

Backwards Incompatible Changes
- Renamed force parameter for `loads` from value to magnitude to avoid naming conflict

- Adding unit tests to validate results for multiple cases
- Formatting improvements
- Added additional parameters
- Re-factored loads and reaction properties
- Fixed issue where reaction would not calculate properly when force was
located directly over a reaction