Concept for QuArrays.

This is a first draft of `QuArrays` which would be the basis type for quantum states and operators. It is a somewhat biased proposal since it is mainly based on my needs for QuDOS.jl. However, I hope it is Ok as a starting point for further discussions.

The main idea is similar to `NamedArray`s : a `QuArray` consists of a container for the coefficients (matrix elements) and a field for storing information about the basis. The latter has its own (abstract) type(s), but for many (?) applications `FiniteBasis` might be sufficient which just stores the number of states in the basis. Additionally, there is `FinitePBasis` which represents a basis of product states (to describe composite systems). Instead of the basis types one could probably also use the spaces introduced by Jarrett? Maybe we should also allow for a separate basis for each dimension?