Some new Empty ITensor constructors

[mtfishman]

This is a proposal to use ITensor{N}() as an alternative syntax for constructing an ITensor with Empty storage and N indices. This could also be written as emptyITensor(Val{N}) or emptyITensor(Order{N}) to make it clearer that an Empty storage is being created (where struct Order{N} end could be a type we introduce like Val that is used as a compile-time order of a tensor).

For example, ITensor{Any}() could be another way to write emptyITensor(Any). Also, ITensor{3}() could be a syntax for making an ITensor with Empty storage of order 3 with unspecified indices. This could be useful for doing in-place operations where you don't want to specify the indices:

A = ITensor{2}() # ITensor with Empty storage and 2 unspecified indices
i = Index(2)
A[i => 1, i' => 2] = 2.5   # This makes an ITensor with indices (i, i')

and:

A = ITensor{3}()
i = Index(2)
for _ in 1:5
  A .+= randomITensor(i,i',i'')
end

(Note this builds off of PR #348).

[emstoudenmire]

Yes, I think good to have these. It’s nice we can strike a balance between having to specify the exact indices of an empty ITensor versus having to just use ITensor{Any}() for every situation where they aren’t known ahead of time.

[mtfishman]

Closed by #641.