Add tool for generating steady-state land use for model initialization

[ckoven]

This PR adds the script file calculate_luh2_secondary_agedist.py in src/constantlanduse. The purpose of this is to calculate disturbance rates that will give steady-state secondary land age distributions that approximate what would happen if a transient LUH run starting at the beginning of the LUH period were run.

I originally wrote this as a jupyter notebook in the repo https://github.com/ckoven/constantlanduse. @maritsandstad rewrote as a standard python script and I further edited to make better use of command-line arguments. The current version takes three arguments:

1. path to file of the LUH data that has been processed for FATES
2. year to calculate steady-state for
3. grid name (used only in the output filename)

The script was tested in making the initial files for the TRENDY runs I ran recently. It could probably use quite a bit of cleanup and testing.

[samsrabin]

CTSM will need this for surface dataset generation, so @ekluzek and @slevis-lmwg should review for possibility of chaining this in to that workflow.

This also needs a README added, then someone totally naïve to this should test it using just the README.

[samsrabin]

@glemieux Please also add a CTSM issue about this with the "external" label, just for tracking purposes.

[glemieux]

@glemieux Please also add a CTSM issue about this with the "external" label, just for tracking purposes.

Associated ctsm issue is now ESCOMP/CTSM#3505. Note that I made it a child issue to the broader ESCOMP/CTSM#3101 issue.