The BulkCosseratDensity class implements three types of curvature
tensors. Previously, the preprocessor was used to select the one
to be actually used.  This patches removes the preprocessors checks
and implements run-time switches instead.

Currently the user code that plugs together the assembler for a
particular problems has to know that ADOL-C is used internally
to compute certain derivatives.  In shows because certain objects
need to be instantiated with adouble as the number type.  One such
example is all objects derived from LocalDensity.

However, if this is to be avoided, then assemblers need to have
a way to turn a (e.g.) 'double' version of a density into an
'adouble' one.  This commit provides this way, by adding a
'makeActiveDensity' method.

Lisa Julia Nebel authored 1 year ago and Sander, Oliver committed 8 months ago

The LocalIntegralStiffness assembles a tangent matrix by
suitably combining derivatives of the energy density with
derivatives of the geometric FE interpolation.  See the
detailed description in dune-gfe-manual.pdf.

This patch also includes a new test in localintegralstiffnesstest.cc.
It checks if the assembled matrix is the same as the one computed
by LocalGeodesicFEADOLCStiffness.

For a 3d Cosserat material I get a speedup of about 3x.