Add multiset-valued tensors #18

I think the next big step after #17 is adding multiset-valued tensors.
Steps:

• Add multisets: these are simply collections where multiplicity counts but not order. I don't think we'll need to add a comparison operator to them, so I think we could just implement them using arrays.
• Add the bar function (or whatever you wanna call it, in the paper it's represented as a bar above its operand), which is a function taking a tensor from A to B and returning a tensor from multisets(A) to multisets(B).
• Make reduction functions reduce multisets into single values. E.g: sum({{1, 3, 3}}) = 7 (where {{1, 3, 3}} is the multiset with 1, 3 and 3).
• With those two things, reductions with index mapping f, tensor U and reduction fn omega, can be implemented as: omega ○ bar(U) ○ f

For a better understanding, please refer to the paper https://github.com/SimplexLab/cten_paper, and especially to the figures.