MPAS lack of optional connectivity vars leads to exception

[erogluorhan]

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Describe the bug There are some connectivity variables such as verticesOnEdge, lack of which shouldn't prevent UXarray from constructing the grid.

To Reproduce Steps to reproduce the behavior:

1. Try opening 3.75km DYAMOND grid, courtesy of @falkojudt
2. See the exception (i.e. grid type not correct) due to lack of verticesOnEdge
3. You can also observe meshes from the other resolutions (7.5km, 15km, 30km) have this variable, so they can be opened with UXarray.

Expected behavior UXarray.Grid should still be constructed with lack of verticesOnEdge or similar vars.