Add support to store values without reduction in history

[pgrete]

This request came up on the AthenaPK matrix channel.

I think the easiest approach is to

• add a new type that prevents any reduction from happening
• with the callback function that is called by all ranks (so no changes), but for which only the value returned from rank 0 is actually being used (as it's the rank that already does the writing to the output file)

Thus, no additional changes are required and downstream codes only need to ensure that the desired value is properly available (returned) by rank 0.

Here are some pointer on how to tackle the implementation:

• Add bew UserHistoryOperation type, e.g., None here

  parthenon/src/defs.hpp

  Line 168 in 14363c9

  enum class UserHistoryOperation { sum, max, min };

• Expand the results list for that type here

  parthenon/src/outputs/history.cpp

  Line 58 in 14363c9

  UserHistoryOperation::sum, UserHistoryOperation::max, UserHistoryOperation::min};

• Ensure that the reduction are not called for the None type here

  parthenon/src/outputs/history.cpp

  Line 151 in 14363c9
• Add documentation around here

  parthenon/doc/sphinx/src/interface/state.rst

  Line 197 in 14363c9

  Currently supported reductions are

  especially on highlighting that only rank 0 data is being used for this type
• (ideally) add/extend a test case (once we're here I'm happy to provide more pointers)

Unrelated: I just noticed that we seem to only support Real values in history files. Does this cause any issues for anyone at the moment?