feat: add keep_shutter_open_across to MDASequence

[tlambert03]

This is alternative to #97, that adds a keep_shutter_open logic to the MDA sequence. The logic is described in the docstring:

A tuple of axes str across which the illumination shutter should be kept open.
Resulting events will have keep_shutter_open set to True if and only if
ALL axes whose indices are changing are in this tuple. For example, if
keep_shutter_open_across=('z',), then the shutter would be kept open between
events axes {'t': 0, 'z: 0} and {'t': 0, 'z': 1}, but not between
{'t': 0, 'z': 0} and {'t': 1, 'z': 0}.

closes #97
closes #3

sidenote: this also opens up a possibility for a generalization of both this and autofocus plan, which would be general functions that take in the current event and (optionally) the upcoming event, and return either a modified current event, a new event or sequence of events to insert, or block yielding of the current event

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.08% 🎉

Comparison is base (6585cb4) 96.41% compared to head (cae32a4) 96.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
+ Coverage   96.41%   96.49%   +0.08%     
==========================================
  Files          14       14              
  Lines         808      827      +19     
==========================================
+ Hits          779      798      +19     
  Misses         29       29              

Files Changed	Coverage Δ
src/useq/_iter_sequence.py	100.00% <100.00%> (ø)
src/useq/_mda_sequence.py	98.46% <100.00%> (+0.10%)	⬆️

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[fdrgsp]

I like it, better approach than #97.