Extract cubewrite() time dimension fix function

[truth-quark]

This issue has several parts relating to the time coord dimension modification:

um2nc-standalone/umpost/um2netcdf.py

Lines 383 to 414 in 1c61fd9

	try:
	# If time is a dimension but not a coordinate dimension, coord_dims('time') returns empty tuple
	if tdim := cube.coord_dims('time'):
	# For fields with a pseudo-level, time may not be the first dimension
	if tdim != (0,):
	tdim = tdim[0]
	neworder = list(range(cube.ndim))
	neworder.remove(tdim)
	neworder.insert(0, tdim)
	if verbose > 1:
	print("Incorrect dimension order", cube)
	print("Transpose to", neworder)
	cube.transpose(neworder)
	sman.write(cube,
	zlib=True,
	complevel=compression,
	unlimited_dimensions=['time'],
	fill_value=fill_value)
	else:
	tmp = iris.util.new_axis(cube, cube.coord('time'))
	sman.write(tmp,
	zlib=True,
	complevel=compression,
	unlimited_dimensions=['time'],
	fill_value=fill_value)
	except iris.exceptions.CoordinateNotFoundError:
	# No time dimension (probably ancillary file)
	sman.write(cube, zlib=True, complevel=compression, fill_value=fill_value)

1. Decouple sman.write() from the time conversion logic
2. Extract the time dimension fix to a function
3. Merge all writes to a single function call

Unit testing is covered in #143.