[test][pnm] add solid heat conduction test

dumux/flux/porenetwork/grainfourierslaw.hh

test/porenetwork/CMakeLists.txt

test/porenetwork/solid_energy/CMakeLists.txt

test/porenetwork/solid_energy/main.cc

test/porenetwork/solid_energy/params.input

test/porenetwork/solid_energy/problem_solid.hh

test/porenetwork/solid_energy/properties.hh

...references/test_pnm_solid_energy_stationary-reference.vtp

.../references/test_pnm_solid_energy_transient-reference.vtp