Submodule plots is in `__all__` but not imported in `__init__.py`

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Hi,

This might be deliberate but it seems that the submodule `plots` is not imported in the `__init__.py` but is included in `__all__`.
I can see there are some optional dependencies for some of the functions but their imports are only inside the concerned functions.

That means that if someone wants to use the `plots` submodule they would need to explicitly import it, which seems a bit strange when the documentation has this:

```python
import cmocean
cmocean.plots.plot_gallery()
```

Which currently gives:

```python
AttributeError: module 'cmocean' has no attribute 'plots'
```

Start in the package __init__.py and compare its __all__ entry for plots with the documented import example. Verify the current behavior of `import cmocean` followed by `cmocean.plots.plot_gallery()`. Done means the documented access no longer raises AttributeError.

Written by the indexing model from the issue text.