Enable using multiple uncertainty estimates per value

Some values can be accompanied by uncertainty estimates from multiple sources. It would be helpful to have the ability to add not just a single uncertainty value, but an ordered list of uncertainty values, each of which gets propagated in turn.

This would imply that we are able to do operations between values with a list of uncertainties:
a = uncertain(42.0, [5.0, 4.0])
b = uncertain(36, [12, 13])
where it has to be checked that the uncertainties list length is identical between subsequent pairs.

In operations between mixed types:
a = uncertain(42.0, [5.0, 4.0])
b = uncertain(36, 12)
the propagation would happen with each list item from a in turn with the uncertainty from b.