Helper functions

Some helper functions that would be beneficial:

• Quantize a value (strictly Operation.quantize_input can be used, at least if made a staticmethod)
• Insert zeros in arrays (for upsampling) !285 (merged)
• Interleave several arrays (to create a single signal after polyphase decomposed upsampling) !280 (merged)
• Polyphase-decompose a signal !285 (merged)
• Downsample/compress a signal l[P::M] (P = phase, M = factor) !280 (merged)