FTR add fitvd pipeline step

[sidneymau]

Add pipeline step to run shredx and fitvd. Note that this addresses #26

Example eastlake config block:

[...]

fitvd:                                                                                                                                                                                                                     
  conda_prefix: "/pscratch/sd/s/smau/conda/des-y6-fitvd-scratch"
  n_jobs: 128 
  det_band: "r" 
  bands: ["g", "r", "i", "z"]

[...]

[sidneymau]

The conda environment should contain roughly the following:

name: des-y6-fitvd
channels:
- conda-forge
- default
dependencies:
- des-fitvd
- shredx
- numpy=1.23.5

Note that I found it important for this to be installed in SCRATCH at NERSC to reduce overhead when spawning all of the subprocesses that run the commands here.

[beckermr]

LGTM! Are you ready to merge this or is there additional testing to do?

[sidneymau]

Sorry for the delay. Have now tested this in the context of an end-to-end eastlake config and smoothed out some bugs