Warn about setting a GDAL cache max if probably on a SLURM node

[phargogh]

After some time away from Sherlock, I wanted to run a simple pygeoprocessing function call on a relatively small raster but found myself exhausting memory. Of course, I had forgotten that I was running this within a SLURM job, which may have some extra identifying features available, such as environment variables.

It would be really great to be able to, at import time, log a warning or something that we are running in a slurm job and that gdal's cache max is unset, defaulting to a very large memory limit. Even better would be if we could detect the slurm job memory limit and compare it to the GDAL cache limit, but even a simple warning would be great.