Add UnstructuredTriangleGrid

[sdrave]

This adds pymor.grids.unstructured.UnstructuredTriangleGrid as a base class for unstructured 2d grids with triangle reference elements, partially resolving #9.

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Repository health decreased by 0.00% when pulling 319e6f3 on unstructured_grid into 3fa4adf on master.

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Repository health decreased by 0.00% when pulling 38fa988 on unstructured_grid into 3fa4adf on master.

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[ftalbrecht]

I will not have the time to look into the changes this week, but it looks good to me. Independently of the UnstructeredTraingleGrid I really like the changes concerning the bounding_box and the reference elements; I think

if grid.reference_element in (triangle, square):

is much nicer than

if isinstance(grid, (TriaGrid, RectGrid)):

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Repository health increased by 0.00% when pulling e80122a on unstructured_grid into 3fa4adf on master.

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