Reduce the time taken to import cf #902

Importing cf is quite slow ... on my local computer it takes 2 seconds.

There seem to be two main culprits for the slow import:

• Doc string rewriting
  • At import time, every docstring (of which there are currently 5569) is inspected for doc string substitutions, replacing any that are found
• Importing external modules that themselves have a slow import
  • The main problems here are dask, scipy, s3fs, zarr, h5netcdf, uritools, netCDF4

These can be improved on:

• Doc string rewriting
  • Handled in cfdm : Reduce the time taken to import cfdm cfdm#361
• Importing external modules that themselves have a slow import
  • Move the imports to run time, rather than import time. Also import a lot of this from cfdm. Many will not ever get imported, and when they do, the time is usually negligible compared to the operation being run.

Results

By applying these changes, my local import time reduces to 0.25 seconds (from 2 seconds - a factor of 6 speed-up).

PR to follow