Check that a value is a list, coercing strings into a list with one member. Useful where users forget the trailing comma that turns a single value into a list.
You can optionally specify the minimum and maximum number of members. A minumum of greater than one will fail if the user only supplies a string.
>>> vtor.check('force_list', ())
[]
>>> vtor.check('force_list', [])
[]
>>> vtor.check('force_list', 'hello')
['hello']