Handling string-based dimensions

[dwest77a]

Xarray behaviour is to collapse these into dtypes that then apply to the specific variable.

This behaviour either requires revisiting the CFA engine to include this xarray collapsing behaviour, or to revisit the creator tool to see if encoding the dimension differently is the solution.

[dwest77a]

Needs a revisit to the creator as the string64 or similar dimensions are currently included in the fragment map with a size of 1, but may also need revisiting the xarray engine to find this collapsing behaviour