Pyseismosoil

What's Changed
* Repo cleanup by jsh9 in https://github.com/Caltech-geoquake/PySeismoSoil/pull/20
* Use static typing; update doc bldg workflow by jsh9 in https://github.com/Caltech-geoquake/PySeismoSoil/pull/21


**Full Changelog**: https://github.com/Caltech-geoquake/PySeismoSoil/compare/v0.5.3...v0.5.4

What's Changed
* Included more flake8 linters and updated code styles accordingly (https://github.com/jsh9/PySeismoSoil/pull/10)

(There are no actual code changes in this release.)

**Full Changelog**: https://github.com/jsh9/PySeismoSoil/compare/v0.5.2...v0.5.3

What's new:

- Switched to tox to manage and run testing and linting

There are no actual code changes in this release.

What's new:

- Updated a docstring
- Updated some examples

There are no actual code changes in this release.

- The formula that calculates ρ (mass density of soils) from Vs (shear wave velocity) is updated: log10 is used instead of log (natural logatirhm). See the updated code [here](https://github.com/jsh9/PySeismoSoil/blob/56609cd124aabe664e78aa679c8dfcdbe10224b2/PySeismoSoil/helper_hh_calibration.py#L760-L765). This is to correct a mistake in notation in Mayne et al. (1999).
- Python 3.6 is no longer supported.