Skydy

This release has added some system analysis functionality, including:
- Controllability of a system, both linear and nonlinear;
- Stability of a system;
- Poles (or eigenvalues) of a system.

Various documentation updates along the way too.

This release is less functional, but extremely practical.

Comprehensive docstrings were added to all classes and methods as a pre-requisite to automatically generate documentation using Sphinx.

The infrastructure was included to generate this documentation, as is available now available on read the docs -- of which the documentation will reflect the latest released version of this repository.

Some other changes were made to the Joint and DOF constructors, but these are minor and are outlined in the documentation ;)

Coverge at 99% -- report attached.
PDF of documentation included.
[Coverage report.pdf](https://github.com/smkyle90/skydy/files/6644194/Coverage.report.pdf)
[skydy-readthedocs-io-en-latest.pdf](https://github.com/smkyle90/skydy/files/6644268/skydy-readthedocs-io-en-latest.pdf)

The first release of the Skydy package. -- a programmatic way to describe inter-connected mechanical systems and get their equations of motion, as well as other useful pieces of information.

All instructions are contained in the package's README.

There is a comprehensive TODO list that will form the basis for future versions and releases.