feat: formalizing point-visiting strategies

[tlambert03]

adds

• useq.TraversalOrder - to dictate how random points are traversed
• addes a nearest neighbor and 2-opt algorithm, (for slightly better path choosing)
• allows you to specify the starting index
• adds plotting to all points plans:

import useq

pp = useq.RandomPoints(
    num_points=100,
    max_height=6000,
    max_width=6000,
    order=useq.TraversalOrder.TWO_OPT,
    random_seed=1,
    shape="rectangle",
    start_at=10,
    fov_height=300,
    fov_width=300,
    allow_overlap=False,
)
pp.plot()

two-opt:

nearest neighbor:

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 74.07407% with 49 lines in your changes missing coverage. Please review.

Project coverage is 94.05%. Comparing base (19cc774) to head (1caaf46).

Files	Patch %	Lines
src/useq/_plot.py	0.00%	43 Missing :warning:
src/useq/_grid.py	95.00%	2 Missing :warning:
src/useq/_plate.py	0.00%	2 Missing :warning:
src/useq/_position.py	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #177 +/- ## ========================================== - Coverage 95.90% 94.05% -1.86% ========================================== Files 16 18 +2 Lines 1075 1177 +102 ========================================== + Hits 1031 1107 +76 - Misses 44 70 +26 ```

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[tlambert03]

this is going to temporarily break one of the new point plan selectors in pymmcore-widgets, because requires a max_width > 0