Change functions to open files with the more generic `mule.load_umfile` instead of the specific classes

[atteggiani]

Overview

Currently the functions open the UM files using specific mule classes based on the type of the fields file (e.g. mule.DumpFile.from_file()).

This limits the applicability of the function only to the specific UM fields file.

Solution

In the cases when a function might be used with multiple UM files (e.g. restart dumps, ancillaries, output fieldsfiles), the function used to open the file should be changed to mule.load_umfile().