No CRDS write

[TheSkyentist]

As discussed in #182, grizli attempts to write to a CRDS file, which can cause issues on systems with shared CRDS files that are write protected. The proposed solution was to copy over the file to the local CONF.

However this does not work due to a big where the entire file path is .lower()ed, rather than just the filename (and so only works on case-insensitive filesystems). I fix the issue by only .lower()ing the filename, not the entire path (a470bff).

To solve the original issue I perform a check to see if the CRDS file is writeable. If not, the CRDS file if copied to the local CONF directory and the changes are applied there (perhaps this should be the default behaviour). Any subsequent processing can then use the writeable version that is created (6d45ab6).

Finally, in the case where the original CRDS file is updated, we must check if the local CONF version is up to date. If out of date, we redo the copying step (97ff0fc and f49f6ee).

These fixes were developed after working through this with @lboogaard with input from @rameyer and @jdavies-st.

[gbrammer]

Could you add a test for the presence of the CRDS_READONLY_CACHE environment variable and also do the thing to write the file in CONF if it's found and True?

[TheSkyentist]

Good idea! Now implemented in f9b86c3.

[gbrammer]

Looks good now. Thanks!