fix: MDASequence iter_sequence method

[fdrgsp]

This PR fix a bug in the MDASequence iter_sequence method.

Currently, if we iter through a MDASequence where we specify channels (> 1) in the main sequence and channel(s) + any plan in the sub-sequence like the one below...

mda = MDASequence(
      axis_order="tpgcz",
      channels=["Cy5", "FITC"],
      stage_positions=[{"sequence": {"channels": ["DAPI"], "z_plan": {"range": 2, "step": 1}}}]
)

...we duplicate the number of events (duplication depends on the number of channels that are specified in the main sequence.). This is the current result:

index: {'p': 0, 'c': 0, 'z': 0}  channel: DAPI
index: {'p': 0, 'c': 0, 'z': 1}  channel: DAPI
index: {'p': 0, 'c': 0, 'z': 2}  channel: DAPI
index: {'p': 0, 'c': 0, 'z': 0}  channel: DAPI
index: {'p': 0, 'c': 0, 'z': 1}  channel: DAPI
index: {'p': 0, 'c': 0, 'z': 2}  channel: DAPI

with this fix, we make sure that the the sequence is not executed n times.

index: {'p': 0, 'c': 0, 'z': 0}  channel: DAPI
index: {'p': 0, 'c': 0, 'z': 1}  channel: DAPI
index: {'p': 0, 'c': 0, 'z': 2}  channel: DAPI

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (03d1b76) 96.11% compared to head (5ae83d3) 96.11%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #93   +/-   ##
=======================================
  Coverage   96.11%   96.11%           
=======================================
  Files          10       10           
  Lines         617      618    +1     
=======================================
+ Hits          593      594    +1     
  Misses         24       24           

Impacted Files	Coverage Δ
src/useq/_mda_sequence.py	97.95% <100.00%> (+0.01%)	⬆️

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