Improve Documentation

[gutmann]

This is really an ongoing challenge, but I'd like to make a concerted effort to get all routines documented and create more detailed doc in readthedocs on the usage of ICAR. ALL are welcome to help with this.

[gutmann]

• Need to document how to add a variable to ICAR (e.g. where to add meta data, kVARS,...)
• Need to document how to add a physics model (e.g. request_vars, memory + tile indices)