Add support for METOP data sets archived at NCEI

[DCarSunDr]

Added "ncei_l1b_remote_data_dir" to POES config.py to give option of loading from current remote location.

Modified POES 'load.py' to add a "ncei_l1b_server" option to use the current remote location, modify the path format correctly if it has been set, and to load from netcdf files instead of cdf files.

Modified netcdf_to_tplot.py to handle some differences between the GOES format and the METOP format for netcdfs, for factors such as identifying units for the time variable, and fill values which weren't getting applied correctly.

Added test cases in POES test.py

Closes #1346

[jameswilburlewis]

This one was a little tricky to merge, because your branch was out of sync with master -- the poes/tests.py script had been renamed in master to poes_tests.py, and that caused a conflict that had to be manually resolved. But once I renamed the file in your branch, I was able to resolve the conflict in the Github editor and finish the merge.

[DCarSunDr]

Yeah, I only realized that after I went to publish the branch that it was derived from an earlier branch unfortunately. I'll make sure future branches are just created from the most recent master branch.

[DCarSunDr]

Thank you for correcting the name!