Add basic batching function

[sjavis]

Adds a basic batching function. It creates a directory for each batch, retrieves / preprocesses data, then runs the tracker.

The function takes these arguments:

• The tracker object to use.
• The number of batches to perform.
• The names of the input files to pass to the tracker during each batch. It accepts * wildcards to match multiple files.
• A function to retrieve the data / put it in the batch directory.
• A list of preprocessing steps (the function + a dictionary of arguments to pass when the function gets called). Files in the batch directory can be specified using %BATCH%, e.g. "%BATCH%/input.nc".

It includes options for where to save the outputs, whether to combine the output files (will be implemented in a later PR), and whether to delete the batch directories at the end.

Closes #191

[surbhigoel77]

If the directory is non-empty, it will raise error and will not delete it.

[sjavis]

Thanks for spotting. Fixed in efe7b86.

[surbhigoel77]

Is the comparison between a cf-Field and string legit here?

[sjavis]

As it turns out, no. In that case the equality seems to return the field for some reason which is truthy. I've moved it inside the check for string types in 4b73781.

[sjavis]

This should be ready. Only the last two commits are new. The rest have just been rebased onto main / v1.0.0-rc branch.