Add documentation for class Mask

[spiani]

Currently, the Mask class is one of the most critical components of bit.sea. However, its code lacks proper documentation and could benefit from improved readability.

We should add documentation to make this class more accessible for use in other projects, such as MER.

It may also be sensible to have this class inherit from the Mesh class (see #21), as it provides a similar interface and is well-documented and tested.