constraint argument support for strings

[cpelley]

Currently, constraints are expected to be a list of strings. These should be made to support a single string also (i.e. a single string), since this would be the natural UI to expect.

improver should handle 1 of something rather than enforcing an iterable of that something unnecessarily. This sorta change would be in-line with as_cube and as_cubes that I recently added to improver to achieve much the same for standardising cube/cubelist handling.

Example:

improver/improver/utilities/cube_extraction.py

Line 273 in f6c2578

constraints: List[str],