feat: add RelativePosition instead of GridPosition

[tlambert03]

when looking at #166, I realized that GridPosition is pretty much only there to represent the concept of some "offset" relative to an absolute position. This PR renames GridPosition to RelativePosition, and uses it more braodly

[codecov]

Codecov Report

Attention: Patch coverage is 96.15385% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.60%. Comparing base (c024a68) to head (886a57a).

Files	Patch %	Lines
src/useq/_position.py	92.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #169      +/-   ##
==========================================
- Coverage   95.65%   95.60%   -0.05%     
==========================================
  Files          15       15              
  Lines        1058     1069      +11     
==========================================
+ Hits         1012     1022      +10     
- Misses         46       47       +1     

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[tlambert03]

note @fdrgsp, this PR removes row/col from GridPosition, not for any particular reason, but that i realized it's not actually being used here anywhere... do you have thoughts on where if anywhere those were used?

[tlambert03]

found it :)

________________________________________________________________________________
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/pymmcore_widgets/_mda/_grid_widget.py", line 554, in _move_to_row_col
    if pos.row == row and pos.col == col:
       ^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/pydantic/main.py", line 827, in __getattr__
    raise AttributeError(f'{type(self).__name__!r} object has no attribute {item!r}')
AttributeError: 'RelativePosition' object has no attribute 'row'

[tlambert03]

i added back row/col ... they're not actually used anywhere, so I'm tempted to remove them, but we can leave that for another discussion. I like how this unifies the concept of positions used across useq, so would like to get this in before release. have a look when you can @fdrgsp