- Removing sigma and eps from IPData, now stored in current state and prev state.
 - Remove associated ip getter and setter, now handled through reflections
 - Stress, stiffness tensor, and volumetric body force are now fetched from
   newly updated constitutive data (CD).

Separating the solid material states from integration
point data. Also move all but one functions into the
local assembler, since they are no longer accessing
the integration point data.

Especially the solid_material is now stored once per
element and not in every integration point.

The structure is small and does not require to be stored on heap.

The evaluation of solid density is moved to the
updateConstitutiveRelations.

postTimestepConcrete of local assemblers.

This saves significant (up to 1/2) memory of the integration point data.

Only those mechanics processes which already provide access to material
state variables.

Mostly using clang-tooling for automatic replacement,
though some parts were modified by hand.

boost::none -> std::nullopt
boost::make_optional -> std::make_optional
boost::optional<T>::get() -> std::optional<T>::value()

Passing (yet unused) variables and variables_prev.