Installing Multiple Versions of IDAES

[eslickj]

It is sometimes nice to install multiple versions of IDAES. For example, some models may only work with a particular older version of IDAES. IDAES provided an environment variable IDAES_DATA that lets you set a location for the IDAES data directory. Using directories other than the default can allow you to have multiple versions of IDAES with different configurations and different copies of the binary extensions.

If users use conda or venv to manage environments, the IDAES_DATA environment variable could probably be set by the environment. Users could probably have other setups for this too. This may be less of an issue if we can install binaries with pip, but I'm not optimistic that we'l have that soon.

I propose we add documentation to the installation instructions about managing multiple model installations.