refactor: multi-point-plan

[tlambert03]

This PR goes farther to formalize the relationship between all of the values in a well_points_plan, namely, they are all RelativeMultiPointPlan where

RelativeMultiPointPlan = Union[
    GridRowsColumns, GridWidthHeight, RandomPoints, RelativePosition
]

and they all support

    def __iter__(self) -> Iterator[RelativePosition]: ...
    def num_positions(self) -> int: ...

it also adds fov_height and width to the RelativePosition

[codecov]

Codecov Report

Attention: Patch coverage is 98.07692% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.90%. Comparing base (7cc1c96) to head (8b16c1b).

Files	Patch %	Lines
src/useq/_plate.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
+ Coverage   95.70%   95.90%   +0.20%     
==========================================
  Files          16       16              
  Lines        1070     1075       +5     
==========================================
+ Hits         1024     1031       +7     
+ Misses         46       44       -2     

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