feat: add sequence to Position object + update MDASequence iter_sequence + test [WIP]

[fdrgsp]

This PR introduces a possible way to add an optional MDASequence to the Position object (instead of only the z_plan option) so that specific axes events (t, z, c or g) can be executed only for a specific position. It also updates the MDASequence iter_sequence method to take this Position sequence into account.

marge after #80

[tlambert03]

i think I'm still looking for something a bit more obvious explicit in the tests here: For example, it might reflect the conversation we were having.

i.e.

# test that z positions are overridden (position 1), 
# relatively shifted (position 2),
# or inherited from the parent sequence (position 3). 
MDASequence(
    z_plan={"top": 60, "bottom": 55, "step": 1},
    stage_positions=[
        {"x": 40, "y": 50, "z": 60, "sequence": {"z_plan": {"top": 20, "bottom": 0}}},
        {"x": 60, "y": 70, "z": 80, "sequence": {"z_plan": {"range": 20, "step": 0.5}}},
        {"x": 10, "y": 20, "z": 30},
    ],
)

... and you can do a similar thing for channel, grid, etc...

[tlambert03]

awesome work here @fdrgsp!