ITensor, Tensor, and Array conversion API

[mtfishman]

The Array conversion interface is a bit inconsistent right now. There is the following for trying to avoid copying, which takes the Index ordering as the memory ordering:

• array(::ITensor)

And these where you have to specify the index ordering:

• Array{ElT,N}(::ITensor, ::Index...)
• Array{ElT}(::ITensor, ::Index...)
• Array(::ITensor, ::Index...)

And then some where Matrix and Vector can be used explcitly:

• Matrix{ElT}(::ITensor{2}, ::Index, ::Index)
• Matrix(::ITensor{2}, ::Index, ::Index)
• Vector(::ITensor{1}, ::Index)
• Vector{ElT}(::ITensor{1}, ::Index)
• Vector(::ITensor{1})
• Vector{ElT}(::ITensor{1})

Functions to add would be:

• matrix and vector
• Versions of Array and Matrix where the indices don't have to be specified.
• Versions of array, matrix, and vector where the index ordering can be specified.
• Versions of array, matrix, and vector where the desired element type can be specified.

[mtfishman]

Also, these functions should be documented (some have docstrings, but aren't on the the website).

[mtfishman]

Note that our functions array, matrix, and vector can also be implemented through Julia's convert API, i.e.:

array(A::ITensor) = convert(Array, A)

This API allows more control over the desired element type, etc.

Similarly, we can also define our itensor constructor function as:

itensor(T::Tensor) = convert(ITensor, T)