add __str__ method to Interval object

[alaniwi]

Would be useful to get some info about contents of an Interval object when printed.

clisops/clisops/parameter/_utils.py

Lines 113 to 122 in c6f8ae1

	class Interval:
	"""
	A simple class for handling an interval of any type.
	It holds a `start` and `end` but does not try to resolve
	the range, it is just a container to be used by other tools.
	The contents can be of any type, such as datetimes, strings etc.
	"""
	def __init__(self, *data):
	self.value = parse_range(data, self.__class__.__name__)