Feature/kurganov tadmor

[keiwanjamaly]

Upon this pull request, the Kurganov Tadmor scheme (a Finite Volume scheme) should be implemented into DiFfRG.

• added boundary conditions and parallel executor of all cells
• formulate derivatives and helper derivatives before computing fluxes
• computed reconstruction of derivatives
• implement computation of fluxes, and sources
• implement explicit euler

Road to implicit_euler

• verify get_mass_matrix is implemented correctly
• implement get_sparsity_pattern_jacobian (currently build_sparsity is commented out)
• implement mass helper function
• implement build_sparsity helper function (hopefully it is correct)
• implement jacobian_mass helper function
• implement jacobian helper function

Road to sundials_IDA

• check that reinit_vector is correctly implemented
• verify get_differential_indices (currently returns empty set)
• implement residual_variables (this would require an update of build_sparsity)
• implement jacobian_variables