Check if relative paths exist

[btobers]

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[btobers]

After PR #143 is merged and closed out, merge dev into 138_path_check and then finalize this PR.

[btobers]

@drounce this PR is ready to review. The only change here is that the config manager now validates that a number of relative paths defined in the user's config.yaml file exist. This will now throw an error when the config is loaded upon importing so that debugging something like a broken filepath becomes easier. I have this set up to ignore a number of optional filepaths:

skip_patterns = [
            '*cesm2*',
            '*cmip5*',
            '*gfdl*',
            '*debris*',
            '*h_ref*',
            '*frontalablation*',
            '*snowline*',
            '*meltextent*',
            '*dh_1d*',
            '*dhdt_2d*',
        ]

So this primarily checks for broken paths to the main input datasets (etc. oggm_gdirs, rgi, era5, cmip6, mass balance) and can easily be modified by adding or removing from the list of skip_patterns.