ESM1.5 conversion driver, delete inputs upon successful conversion

[blimlim]

This pull request would close #49.

In most simulations of ESM1.5, the original fields files aren't very useful once they've been converted to NetCDF. This pull request adds a command line flag --delete_ff, -d, to the conversion driver. When included, the conversion driver will delete the input fields files when they are converted.

I've implemented this by adding the deletion as close as possible to the actual conversion:

um2nc-standalone/umpost/conversion_driver_esm1p5.py

Lines 156 to 160 in 379001b

	try:
	um2netcdf4.process(fields_file_path, nc_write_path, ARG_VALS)
	succeeded.append((fields_file_path, nc_write_path))
	if delete_ff:
	os.remove(fields_file_path)

This does bury a bit more I/O in the convert_fields_file_list, but I think it seems safe to have it there to try and ensure that the only way a file can be deleted is if the conversion runs without any errors.

I also modified the convert_fields_file_list tests to check that file deletion is run when requested, and is not run when it is not requested or if there are any errors.

Let me know if these changes look ok, any feedback/suggestions are welcome!

[truth-quark]

Looks good to go!