Move terrautils technical documentation to readthedocs and create tutorials

[dlebauer]

Follow standard practices for a python library.

This is technical documentation for developers and end users wishing to use the library to access data.

A separate goal is to provide quick start tutorials. These can go in github.com/terraref/tutorials. In any case, we should follow a coherent scheme that addresses multiple type of end users and use cases like the one presented here https://www.divio.com/blog/documentation/: