fix: fix skip channel in iter_sequence + tests

[fdrgsp]

This PR fixes a bug in the MDASequence iter_sequence method and adds more tests.

Currently, if we run a sequence with multiple channels and a grid_plan (or z_plan or time_plan) in a position sequence (like the one below), we get ONLY ONE channel through the sequence:

mda = MDASequence(
    channels=[
        {"config": "Cy5", "exposure": 10},
        {"config": "FITC", "exposure": 10}
    ],
    stage_positions=[
        {"x": 0, "y": 0, "sequence": {"grid_plan": {"rows": 2, "columns": 1}}}
    ]
)

running:

for e in mda:
    print(e.index)

current output:

{"p": 0, "c": 0, "g": 0}
{"p": 0, "c": 0, "g": 1}

with this fix, we get both the channels as expected:

{"p": 0, "c": 0, "g": 0}
{"p": 0, "c": 0, "g": 1}
{"p": 0, "c": 1, "g": 0}
{"p": 0, "c": 1, "g": 1}

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3a737f9) 96.11% compared to head (3f37c3a) 96.11%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #85   +/-   ##
=======================================
  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%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.