laplacian should skip non-length dimensions

[dopplershift]

laplacian is implemented naively and just sums the second derivative across the selected dimensions, which is all of them by default. When given an xarray, we could:

1. skip any dimension that doesn't have dimensionality of length.
2. skip any dimension that has dimensionality of time (what about pressure?)

Noted in this Stack Overflow question. I'm not sure if there are downsides, but the current failure mode of a DimensionalityError is a bit tricky to decipher.

We should also update the docs to reflect the limitations and added behavior.

[Z-Richard]

I would vote for (1) because people will likely use laplacian to compute QG-related stuff in a meteorological setting (at least that's the only place I recall laplacian appears). Calculating laplacian in the vertical coordinate is generally less useful. If there is no dimension with a dimensionality of length, I think the function could return a tuple of second derivatives and warn the user that it cannot proceed any further.