Remove MeshSubsets

DOF: partial_sum
BC: iota

The creation of a mesh subset via intersection is
split into two functions; the first one handles the
nodes intersection (and the resulting memory is managed
by the current scope), and in the second step the
actual MeshSubset object is created, w/o any memory
management obligations.

The function is independent of the MeshSubset.

MSS being the mesh subsets.

[PL] Use std::function to give the flux calculation to CalculateSurfaceFlux.

Remove unneeded Base ns specifier

This reverts commit 0a06295b.

[web] Add xsd files into images/xsd.

[web] Marketing / features index page

Reorderd features a bit.
Uploaded precompiled JS and CSS.

Fixed  the calculation of the Darcy velocity in staggered TH

processes of HT in the local assembly.

Balance for HT process