MIMO Transfer data structure

[ilayn]

Currently MIMO transfer num den data is kept in list of lists. This is good for quick access with different lengths of entries. For example, if some element has only 1 and the other element has s^3 + 5s^2 - 4s -7
and they can be kept as [[1], [1, 5, -4, -7]. But for some operations walking over list of lists is too slow and inconvenient. For example adding a 3D zero padded numpy array is much easier to multiply with a scalar etc.

A MIMO transfer object can hold both the list of lists and also the 3D array version. Hence add NumPy arrays.