Adding new scaling from Yamauchi and Takei 2024

[chrishavlin]

This adds the new Yamauchi and Takei 2024 scaling for super-solidus melt effects.

The initial implementation of the pre-melt scaling actually included placeholders for some constants, so it was very easy to use the existing function by adding a new flag. To use the new scaling, you add the same xfit_premelt to the methods list and then set the new include_direct_melt_effect flag to 1:

  VBR.in.anelastic.methods_list={'xfit_premelt'};
  VBR.in.anelastic.xfit_premelt.include_direct_melt_effect = 1;

For now the default is include_direct_melt_effect=0, but eventually it may make sense to have the default set to 1.

This initial push does run but I haven't validated anything.

Still to do:

• [x] add tests
• [x] update docs
• [x] validate (the paper includes a zenodo link to data)

Closes #85