Allow data to be omitted from netCDF files during cfdm.write

[davidhassell]

It would be useful to be able to not write to disk the data from selected construct types.

This would not affect the amount of netCDF variables and dimensions that are written to the file, nor the netCDF variables' attributes, but would not create data on disk for the requested variables thus resulting in a smaller file than it otherwise would have been.

When the new file is read then the data of these variables will be represented by an array of all missing data.

This will be useful for cases when there is a need to store a file's metadata structure for inspection, but the data values are immaterial.