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

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:

import cmocean
cmocean.plots.plot_gallery()

Which currently gives:

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

从 package init.py 开始，将其中针对 plots 的 all 条目与文档中的 import 示例进行比较。验证先执行 import cmocean，再执行 cmocean.plots.plot_gallery() 时的当前行为。当文档中的访问方式不再引发 AttributeError 时即表示完成。