feat: add tile dimension

[fdrgsp]

This PR adds the tile dimension to enable grid acquisition experiments.

TODO:

• TileFromCorners: find how to define the top_left starting position.
• find a way to discriminate between a "general" tile_plan and a tile_plan specified in the Position object. (NOTE: the Position class allows to also set the z_plan but we are not accessing it at the moment, we should fix this as well).
• update tests.

[codecov[bot]]

Codecov Report

Base: 88.07% // Head: 89.09% // Increases project coverage by +1.01% :tada:

Coverage data is based on head (17ef625) compared to base (82e4b08). Patch coverage: 93.44% of modified lines in pull request are covered.

:exclamation: Current head 17ef625 differs from pull request most recent head 8f0ebe3. Consider uploading reports for the commit 8f0ebe3 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #70 +/- ## ========================================== + Coverage 88.07% 89.09% +1.01% ========================================== Files 9 10 +1 Lines 436 550 +114 ========================================== + Hits 384 490 +106 - Misses 52 60 +8 ``` | [Impacted Files](https://codecov.io/gh/pymmcore-plus/useq-schema/pull/70?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymmcore-plus) | Coverage Δ | | |---|---|---| | [src/useq/\_mda\_event.py](https://codecov.io/gh/pymmcore-plus/useq-schema/pull/70?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymmcore-plus#diff-c3JjL3VzZXEvX21kYV9ldmVudC5weQ==) | `89.79% <ø> (ø)` | | | [src/useq/\_tile.py](https://codecov.io/gh/pymmcore-plus/useq-schema/pull/70?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymmcore-plus#diff-c3JjL3VzZXEvX3RpbGUucHk=) | `92.70% <92.70%> (ø)` | | | [src/useq/\_mda\_sequence.py](https://codecov.io/gh/pymmcore-plus/useq-schema/pull/70?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymmcore-plus#diff-c3JjL3VzZXEvX21kYV9zZXF1ZW5jZS5weQ==) | `88.00% <95.23%> (+0.68%)` | :arrow_up: | | [src/useq/\_\_init\_\_.py](https://codecov.io/gh/pymmcore-plus/useq-schema/pull/70?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymmcore-plus#diff-c3JjL3VzZXEvX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | | | [src/useq/\_position.py](https://codecov.io/gh/pymmcore-plus/useq-schema/pull/70?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymmcore-plus#diff-c3JjL3VzZXEvX3Bvc2l0aW9uLnB5) | `96.42% <100.00%> (+0.13%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymmcore-plus). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymmcore-plus)

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[tlambert03]

Will want to see coverage of _tile.py get above around 90% before we merge this: https://app.codecov.io/gh/pymmcore-plus/useq-schema/pull/70