Create central place for IDAES intersphinx mapping

[dangunter]

For the docs, intersphinx is a tool to get aliases for other projects' documentation and allow explicit and automatic linking between them. There are a number of IDAES PSE projects now so it would seem simpler to put the URLs for their documentation and a canonical name for them all in one place. Logically, for all the ones that use idaes-pse (WaterTAP & ProMMiS) this can be done as a Python module inside the idaes-pse package since they need to import it anyways to build the docs.

[dangunter]

Too complex