refactor channel chunking

[JennySmith888]

refactor channel chunking to incorporate hardware requirements for capture as they differ from user requested channel subset.

• fixes driver bug where groups was not made an even number
• new helper functions to convert groups <--> channels for phase and IQ
• renamed keep_channels to keep_groups because it returned groups not channels (I can undo this without protest)
• chunking for capture size takes into account required hardware capture size and chunks accordingly
• if pynq buffer can be sliced, it is and the requested data is returned without a copy, if it cannot be sliced, the requested data is copied to PS RAM