Add fill metadata to files and fix yamls as second observation is now rejected

[Michael Cooke (mikecooke77)]

This PR fixes #277 failing tests that have been noted in the issue.

The following changes have been made:

1. Update the python script which produces the files to add a fill value for all the variables in the nc4 files. This script has then been run to recreate the nc4 test files.
2. The second location for all the global varobs writer is now rejected and this is reflected in the updated yamls which have reference values.

[Matt Shin (matthewrmshin)]

Tested in my environment. All looking good. https://cylchub/services/cylc-review/taskjobs/matthew.shin/?suite=mobbm-oi278%2Frun1

[Chris Thomas (ctgh)]

Thanks for the comprehensive change!