[solver] Unify preconditioner configuration file input variable

[maxfirmbach]

Description

We currently specify four different input file names for preconditioner configurations, namely:

• AMGNXN_XML_FILE
• TEKO_XML_FILE
• MUELU_XML_FILE
• IFPACK_XML_FILE

It would be great to merge these four parameters into one. The all do exactly the same, storing the path/name to a file. We could introduce preconditioner_file or similar to replace the four above. I would also not specify the file type in the variable name, because that might change over time.

Possible Solution and Definition of Done

• Replace and unify the four input variables above with one consistent one.
• Change all test cases and tutorials to use the new variable.
• Make things work and ctest.

Interested Parties

@mayrmt @reginabuehler (I put you both here as this is more for user experience / ease of usage e.g. maybe tutorial related)

[mayrmt]

While at it, why not also change the SOLVER_XML_FILE accordingly?

[maxfirmbach]

Yes, could be done in the same step.