[FEATURE] Improve Stacks

[henryiii]

Several improvements to Stacks should be added.

Scaling

This maybe should work:

stack * vector

If these are the same length, then each histogram should get scaled (and other operations). Also true for inplace. Maybe dicts should be supported too?

Projections

Operations on a stack should fall through to the histogram, such as projection.

stack = hist.stack("cat")
new_stack =stack.project("x")
# Should be the same as
new_stack = hist.project("cat", "x").stack("cat")

This probably should go for other operations - slicing would be very nice, but does interfere with accessing the stack itself. Maybe we could have a proxy object? Say stack.hists* would apply an operation on every histogram. So stack.hists[0:2] would take [0:2] on every histogram in the stack and return a new stack? If it returns a simple number, then I guess it returns a list? That's a little more irritation statically, though. stack.list*, perhaps, would always return a list?

Setting

The stacks are not settable, they store tuples and have no setter. This should work:

stack[0] *= 2

The internal storage should be a list, and setting should make sure the new histogram matches the old one in axes. All the histogram mul/add/sub operations should work, including in place ones.

Other

I'm still not very happy with *args being the constructor for Stack, as things like Stack(x for x in xs) is not easy to write (and mappings are also not supported). But maybe too late to change. Tempting to allow both to work if args len is 1 and not an instance of a BaseHistogram.

[henryiii]

Also, stack.name should return and allow the setting of the names.

[henryiii]

Raw setting is the only thing left. s[0] = h, where h gets checked to see if it is valid.

[henryiii]

This is done (at least for the things listed above), but we should add the new stuff to the readme.