Add documentation for reporting units

[dallan-keylogic]

I recently received a question from a user about changing what set of units certain variables are reported in. I am dimly aware that there exists functionality in IDAES to set reporting units, but I am not aware of the specifics. I looked for documentation, but it appears there isn't any. I know there's a function in idaes\core\util\units_of_measurements.py associated with the functionality, as well as a line in the config, but I don't know how either of those work and don't have the time to do a full investigation.

This gap in our documentation needs to be filled by somebody.

[ksbeattie]

Possibly related to: #1433