Extend perturbIC functionality

[atteggiani]

Currently, the perturbIC function can only accept a restart dump file (UM Dump File) as input, and perturb its theta field (STASH itemcode = 4).
Although this meets the most likely use case, the function could be easily extended to:

• work with any type of fields file (Ancil files or Output Fieldsfiles too), perturbing by default its theta field. If the field is not present, an error would get raised.
• accept a STASH itemcode as command line argument, to choose a different field to perturb (for example if user wants to perturb the surface temperature (STASH itemcode = 24) something like --stash-to-perturb 24 could be passed as an argument)
• if the input fields file contains only one single field, automatically perturb that field (even if is not THETA and the --stash-to-perturb has not been passed.

This additions would cover some edge use cases of perturbIC, extending its functionality without degrading its current performance.