FEAT: Build QuakeMigrate travel-time lookup tables from 3-D velocity models

[hemmelig]

Is your feature request related to a problem? Please describe.
Since NonLinLoc is capable of converting 3-D velocity models (such as those created by SimulPS, Simul2000, and FDTomo) into 3-D grid files—which can consequently be fed into the NonLinLoc Grid2Time program—it should be possible to use these velocity models with QuakeMigrate.

Describe the solution you'd like
This Issue would be resolved by the following:

1. Documentation of how to configure NonLinLoc to read each type of 3-D file
2. A function in the quakemigrate.lut module that can build QM compatible travel-time lookup tables from a specified 3-D velocity model

Describe alternatives you've considered
We could alternatively just direct users to handle the conversion from SimulPS files to NonLinLoc outside QM, then use the quakemigrate.lut.create_lut.read_nlloc function to import NonLinLoc travel-time grids into the quakemigrate.lut.LUT class.

Additional context
Documentation from NonLinLoc can be found here: http://alomax.free.fr/nlloc/ (legacy) or http://alomax.free.fr/nlloc/docs (more recent, but incomplete)

Source code for NonLinLoc can be found here: https://github.com/alomax/NonLinLoc

Requested by: @fontiela