Convert AMOC transport files to OceanSITES formatted files.

[eleanorfrajka]

Given that a lot of effort has been put into documenting the OceanSITES data format, a first step towards making AMOC array data easier to use can be through converting original data files into a format conforming with OceanSITES. Some extracts for the format (v1.4) are here: https://ocean-uhh.github.io/oceanarray/oceanSITES_manual.html

Steps might be:

1. Extract the meta data (global attributes + variable attributes) and variable names from a data file (e.g., moc_transports.nc).
2. Check what is missing against OceanSITES required global attributes. (Might not be fixable without involvement from data producer.)
3. Transform data variables as needed. Using coordinates as in OceanSITES recommended format. (Note: probably do not convert PRESSURE to DEPTH since this won't be a 1d array coordinate anymore.)
4. Add attributes to variables as needed using controlled vocabularies for standard_name and compliant formats for the units attribute (Sv is not allowed since it also means "sievert", so instead units of "Sverdrup" or alternatively "m3 s-1" should be used for transports). See units for transport.
5. Name the file according to OceanSITES conventions. See OceanSITES filenames where most of these will be "derived products".

[eleanorfrajka]

On (1), the extraction of metadata and assignment of extra metadata is now handled by the read.<<data_source>>() functions, where <<data_source>> is one of the defined names like "rapid", "osnap", etc.

On (2), I haven't yet cross-checked the mandatory fields in OceanSITES data and on (3), have not shuffled dimensions to be in the OceanSITES recommended order of "T, Z, Y, X".

On (4), we're partway there adding the standard_names from CF-conventions (https://amoccommunity.github.io/AMOCatlas/reference/AC1_variables.html#cf-standard-name-vocabulary), but have not added the vocabulary. Additionally, OceanSITES wants attributes for _FillValue and coordinates if they aren't all the 4 coordinates in the definition.

On (5), haven't started yet, but this will go in the writers.py.