perform L2 projection in grid adaptation on water content, not matric head
The solution DOF vector is currently inserted into the PDELab::adapt_grid() algorithm, which performs an L2 projection onto the new grid. At soil layer boundaries, this might cause abrupt changes in volumetric water content, which might imply that global mass conservation does not hold. The projection conserves the integrated matric head, but not the water content.
To circumvent this, a water content DOF vector has to be created by interpolating from an apropriate DiscreteGridFunction. This vector is then handed to adapt_grid. Afterwards, we interpolate the new matric head solution from the newly received water content.
This issue might be related to the artifacts reported in #8 (closed).
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