Implement DescriptorArray

[henrikjacobsenfys]

For EasyCrystallography and EasyDiffraction, we need to have Descriptors that are arrays. We agreed that it should be named DescriptorArray.

It will inherit from DescriptorBase, and will be implemented as scipp variables using scipp.array.

It will have all the same methods as DescriptorNumbers, including element wise+,-abs, neg.

We will follow numpy conventions. This means that * and / can only be done with Numbers or DescriptorNumbers, and will be element wise.

Matrix multiplication and division will be implemented as well, using Numpy. We will follow the Numpy naming scheme. Numpy Arrays and DescriptorArrays will be accepted inputs.