Show bathymetry plot in notebook + some other fixes

[navidcy]

@aekiss this PR adds the missing thumbnail.

It's not done in an automatic way, I just evaluated that particular cell.

Ideally, we need to resolve #119

Also some fixes in docstrings (eg rectangular_grid) and in the explanations in the demo notebooks.

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[navidcy]

Hm... OK, it's a failed attempt...

[navidcy]

I think I fixed it! Could somebody review this?

@aekiss, here's a preview of the docs showing that the thumbnail is there!

[angus-g]

PR description doesn't capture the fix to rectangular_hgrid. One small typo in your doc changes, otherwise I think they're fine.

I guess this just makes the thumbnails for both notebooks the same. I suppose there's no issue with that (it's better than nothing!)

[navidcy]

With #59 one of the examples will go away anyway

[navidcy]

PR description doesn't capture the fix to rectangular_hgrid.

That's a change only in the comments -- I'm adding a remark

[angus-g]

PR description doesn't capture the fix to rectangular_hgrid.

That's a change only in the comments -- I'm adding a remark

There was also an addition of deg2rad, no?

[navidcy]

In a comment tho, not in source code.