Implement OOP block_structure approach and implement nonsquare complex blocks

[sdahdah]

The current approach to block_structure is that of https://www.mathworks.com/help/robust/ref/mussv.html

If BlockStructure(i,:) = [-r 0], then the i-th block is an r-by-r repeated, diagonal real scalar perturbation.
if BlockStructure(i,:) = [r 0], then the i-th block is an r-by-r repeated, diagonal complex scalar perturbation;
if BlockStructure(i,:) = [r c], then the i-th block is an r-by-c complex full-block perturbation.

but we should do something like

block_structure = [
    dkpy.RealDiagonalBlock(r),
    dkpy.ComplexDiagonalBlock(r),
    dkpy.ComplexFullBlock(r, c),
]

in addition to

block_structure = [
    [-r, 0],
    [r, 0],
    [r, c],
]

The objects should all inherit from something like dkpy.Block or dkpy.Perturbation. Names are open for discussion.

Also, nonsquare full blocks need to be implemented.