documentation should be made consistent

[glennklockwood]

The documentation for IOR and mdtest is pretty inconsistent and scattered across many different files and locations. This meta-issue is meant to track the state of improving the documentation and, for example,

• addressing #138
• addressing #118
• addressing comments in #151
• getting rid of multiple READMEs, USER_GUIDE, etc and creating a single point of truth for documentation
• re-basing and merging https://github.com/glennklockwood/ior/tree/docfixes
• promoting the readthedocs documentation in the repository's top-level README
• adding more Doxygen documentation in the code where appropriate