Use 'donfig' package for global configuration settings

[djhoese]

Feature Request

Is your feature request related to a problem? Please describe.
I could have sworn I made an issue for this already but I can't seem to find it. I created a package called donfig which is based off of dask's global configuration module/object. This would allow easier interactive changes to satpy's global settings (ex. default chunk size, etc). See donfig documentation for more information: https://donfig.readthedocs.io/en/latest/

Describe the solution you'd like
SatPy should create a central config object in satpy/__init__.py that is a donfig.Config('satpy') object. In this we would set defaults for certain settings like default chunk size, plugin /etc directory (previously PPP_CONFIG_DIR), resampling cache directory, other?

Describe any changes to existing user workflow
I think we could make it so that any existing environment variables like PYTROLL_CHUNK_SIZE still have an effect but try to have a deprecation warning if possible. The new environment variable read by donfig would be SATPY_CHUNK_SIZE or whatever we chose to name the parameter. Otherwise, this should only mean new flexibility for the user.

Additional context
We'd have to go through all uses of ppp_config_dir and remove them or deprecate them in favor of the global configuration. Similarly reader's use of the satpy.CHUNK_SIZE would have to be removed.

[gerritholl]

Has anything related to this happened? I'm interested because of #727, because logging messages are hard to process programmatically.