Creation of the polyphase representation of a signal
1d signal (1d vector, n elements)
number of polyphase branches e.g. decimation ratio
Polyphase matrix (m rows, n/m columns)
Creation of the polyphase matrix, with zero padding if necessary. First row of X is x(1), x(1+m), x(1+2m), ..., second row is x(2), x(2+m), etc.