[NDTensors] AllocateData library

[mtfishman]

This introduces the AllocateData submodule library. The goal is to provide a common interface for allocating arrays of different types through the function allocate, for example:

# Allocate undefined data
allocate(Array{Float32}, (2, 2))
allocate(Diagonal{Float32}, (2, 2))

# Allocate zeros
allocate_zeros(Array{Float32}, (2, 2))
allocate_zeros(Diagonal{Float32}, (2, 2))

This is related to the generic_zeros functionality in NDTensors, with a goal of being more principled and general.

It could also be used by the UnallocatedArrays library being introduced by @kmp5VT in #1213.