add RangeMap.__mul__ and tests

[werenike]

Ability to take the product of two RangeMaps, merging their keys and creating tuples from their values.

def test_product():
	rm1 = RangeMap.from_iterable((
		(None, 10, 'a'),
		(10, 25, 'b'),
		))
	rm2 = RangeMap.from_iterable((
		(5, 15, 'c'),
		(15, 25, 'd'),
		(30, None, 'e'),
		))
	assert rm1 * rm2 == RangeMap.from_iterable((
		(None, 5, ('a', None)),
		(5, 10, ('a', 'c')),
		(10, 15, ('b', 'c')),
		(15, 25, ('b', 'd')),
		(30, None, (None, 'e')),
		))
	rm3 = RangeMap.from_iterable((
		(None, 0, 'f'),
		(0, 20, 'g'),
		(35, 40, 'h'),
		))
	assert rm1 * rm2 * rm3 == RangeMap.from_iterable((
		(None, 0, ('a', None, 'f')),
		(0, 5, ('a', None, 'g')),
		(5, 10, ('a', 'c', 'g')),
		(10, 15, ('b', 'c', 'g')),
		(15, 20, ('b', 'd', 'g')),
		(20, 25, ('b', 'd', None)),
		(30, 35, (None, 'e', None)),
		(35, 40, (None, 'e', 'h')),
		(40, None, (None, 'e', None)),
		))

Not sure if the implementation is the best. The class ValueTuple is used to merge tuples of values in products of more than two RangeMaps. Let me know about possibilities for improvement!

[coveralls]

Coverage increased (+0.06%) to 98.023% when pulling d82fe03 on MareinK:rangemap-prod into 84db5bc on mlenzen:master.

[mlenzen]

This is an interesting idea. Regarding implementation, I've been thinking about making RangeMap return views of the underlying data instead of copies (#76). I think that implementation could also be used for this. Since the view would know how many RangeMaps were underlying when each tuple value is created, we wouldn't have to do the nesting and flattening.