refactor!: change axis_order type to Tuple[str, ...]

[tlambert03]

fixes #138

Changes:

MDASequence:
    # this 
    axis_order: str = "".join(AXES)
    # to this
    axis_order: Tuple[str, ...] = AXES

the typing of MDASequence.axis_order was too strict. It's fine to accept a string as input, but it should also be able to accept a sequence of strings, each of which may be more than a single character

@ianhi, can you think of anywhere you're assuming axis_order is a literal string, rather than generic sequence of strings?

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a269e66) 98.28% compared to head (b07287c) 98.28%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #140   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files          14       14           
  Lines         877      877           
=======================================
  Hits          862      862           
  Misses         15       15           

Files Changed	Coverage Δ
src/useq/_mda_sequence.py	98.63% <100.00%> (ø)

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