Implement get_solver for WaterTAP

[bknueven]

Fixes #1348

Summary/Motivation:

Presently, when import watertap runs we change the IDAES default to utilize the ipopt-watertap solver. This can mess with down stream projects as it is a side-effect of importing WaterTAP, e.g., prommis/prommis#52.

This PR would remove this behavior, and replace it with WaterTAP's own implementation of get_solver which works identically to the IDAES version except it returns ipopt-watertap by default.

However, as a consequence, the behavior of get_solver within IDAES has changed for WaterTAP -- it now gets the IDAES defaults when imported from idaes.core.solvers. As a result, we would need to specify the ipopt-watertap solver when calling certain IDAES methods, e.g., f5a55cc, to get the ipopt-watertap solver for initialization. Usually this does not matter, but this PR would make this change on flowsheets that fail without ipopt-watertap for unit model initialization.

Changes proposed in this PR:

• Implement new get_solver method for WaterTAP (e8d7b85)
• Remove code which changes the default IDAES sovler to ipopt-watertap (510b948)
• Update all imports of get_solver to watertap.core.solvers (f628c03)
• Several fixes for affected flowsheets (519a85f, b99eee9, f5a55cc)

Legal Acknowledgement

By contributing to this software project, I agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.