Demonstration of offset feature in XIOS

[NMannall]

Creates two examples to demonstrate the freq_offset feature of XIOS.

• sampling_offset - outputs multiple fields at different offsets to demonstrate the functionality.
• sampling_offset_time_bounds - a simple example that outputs a single field. Added to demonstrate that even when outputting a single field to a file, there is no data in the generated netCDF file to show that an offset was used. It looks like sampling/output started at timestep 0.

[mo-marqh]

i suggest capturing expected or hoped for output, particularly for sampling_offset_time_bounds

it might be worth a comparison, to tease out

there is no data in the generated netCDF file to show that an offset was used

including .cdl plain text content, for netCDF file metadata into the repository for this case, and then using it to demonstrate the checking, may be useful

[NMannall]

I have changed the sampling_offset_time_bounds example (now called sampling_offset_metadata) to use a .cdl file to check the output in two files (one with an offset, one without).

Currently, I check that the metadata matches the .cdl file but only that the field data differs in the two output files. I could change this to test the exact field values in each of the output files instead.