Migrate perturbIC to using mule

[atteggiani]

It would be beneficial for perturbIC to be using mule to process UM fieldsfile data, instead of the custom umfile module.

This will allow standardisation of functions and making testing and implementation of new functionalities simpler.

Important

Currently, mule cannot handle UM fieldsfile timeseries (LBCODE 31320/31323, reference from UM F03 Document) properly. This makes fixing this issue more complex than it could be.
Before switching to mule, we need to make sure a proper way to handle timeseries is found and the "mule-enabled" scripts work well with timeseries.

[atteggiani]

More info about problems with mule: #3 (comment)