Add unstructured mesh output
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With the tutorial, I've noticed that looking at the output with the Value Tool or the Cross section is potentially misleading. In the case of a well, the lowest head can be found right near at the well bore. Depending on the more-or-less accidental placement of grid points, this location is computed, or not.
Before, I figured unstructured grid output is mostly "nice to have" to generate nicer contours, but this is a good argument to invest some time in it.
We could add a "triangular mesh" checkbox under the Domain group, which is check by default. Behind the scenes, we can use pandamesh to generate a triangular mesh which is aware of all element vertices. The maximum grid spacing value can still be used as an upper bound. If the checkbox is unchecked, it will write quads, as it does currently.
QGIS supports both face data (on the centroids) as well as node data, directly on the nodes. In this case, computing data for the nodes is easier: e.g. a well will place a node which will end up in the out grid -- though I'm not sure whether the Value Tool supports node data.
However, for any doublet, something else is required: a node on opposite sides of the doublet line. This can be achieved by buffering the doublet walls by a minor amount and including the vertices of the buffered geometry. Since orthogonality doesn't matter at all, the minimum angle for Triangle can be set extremely low.
This should also make the mesh-versus-raster output less confusing, as they are visually clearly different.