ENH: Implement free partition

[pchanial]

A free partition is different from the stack and chunk partition in the sense that blocks have no shape constraints. The input and output would not necessarily be contiguous in memory.
The input of a free-partition BlockDiagonalOperator and BlockRowOperator would be multivariate.
The output of a free-partition BlockDiagonalOperator and BlockColumnOperator would be multivariate.

Example:

H = BlockColumnOperator([H1, H2])
y1, y2 = H(x)