[WIP] add python script for prefactor scaling

[naliboff]

A python script to correctly convert viscous flow prefactor terms from experimental studies such that they are internally consistent with ASPECT's rheological conventions and assumptions.

@bobmyhill @jdannberg @anne-glerum @cedrict - Is this worth adding in, and if yes does the current structure make sense?

FYI, I am going to add a more detailed description of how the scaling works in #5576, but can also do that here as well. For now, I cited the grain size paper, which provides a description of the conventions in an Appendix.

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