[localoperator] linearelasticity: turn #warning statements into doxygen todos
[localoperators] add new interface class LinearElasticityParameters
[localoperators] update linearelasticity operator to use the new parameter class
[tests] adapt testelasticity to recent changes in linearelasticity operator
[localoperators] linear elasticity: we only support Dirichlet and homogeneous Neumann boundary conditions
[tests] choose better parameters for testelasticity