Improves constness of the local SpatialPosition variables.

Effectively not used anyway.

And remove the corresponding functions in SpatialPosition.

The computation is used only once and had a missing
header include (Eigen/Core). Replaced with more efficient
code directly in FEM.

Sometimes it seems not to be enough.

Remove this

Sort soil and bhe elements only, if they are unsorted

[FBar] temp

from N, dNdx to N_u and dNdx_um respectively.

... such that an isolated small example can be easily downloaded from GitLab as a zip file.