Extract cubewrite() fill value to function

[truth-quark]

Continuing with the overarching task of splitting cubewrite() (#12), fill value functionality can be extracted as a function:

um2nc-standalone/umpost/um2netcdf.py

Lines 166 to 174 in 92096ce

	# Set the missing_value attribute. Use an array to force the type to match
	# the data type
	if cube.data.dtype.kind == 'f':
	fill_value = 1.e20
	else:
	# Use netCDF defaults
	fill_value = default_fillvals['%s%1d' % (cube.data.dtype.kind, cube.data.dtype.itemsize)]
	cube.attributes['missing_value'] = np.array([fill_value], cube.data.dtype)

Tasks:

• Extract code block to separate function
• Add unit tests
• Convert 1e20 fill value to a constant
• Add arg to function signature for custom fill values

For now, let's keep cubewrite()'s steps the same, merging into process() can be a separate refactoring issue.