generalize dimension skipping/decimation

[tlambert03]

as pointed out by @henrypinkard in micro-manager/pycro-manager#266 (comment),

In the Channel object in MDASequence, dimension decimation (acquire every n...) should be generalized to arbitrary axes. one possible (but not ideal) pattern:

class Channel:
    ...
    decimate_axes: Dict[str, int]
    offset_axes: Dict[str, int]

# e.g
{'config': 'DAPI', 'decimate_axes': {'t': 5}}  # acquire DAPI every 5th frame
{'config': 'DAPI', 'decimate_axes': {'z': -1}}  # or something like that ... e.g. don't acquire the full stack
 {'config': 'DAPI', 'offset_axes': {'z': 0.5}}  # focal shift of 0.5 in this channel