Expand file paths uniformly throughout Xopt

[ColwynGulliford]

In running Xopt with a population file I noticed I can't use ENV variables in generator:population_file key in the Xopt yaml file:

generator:
    name: cnsga
    population_size: 320
    output_path: /global/homes/c/cgull12/m3645/cgull12/cooler/pc/in2mg/v2
    population_file: $COOLER/xopt/pc/in2mg/v2/seed_pop0.csv

This causes Xopt to raise a "file not found" exception. If I put the full path in, it works. I am able to use the ENV variable for other paths in the input file though. It would be nice if Xopt allowed this uniformly.