Allow using BackwardEuler integrator instead of VODE

[chongchonghe]

Currently VODE parameters are hard-coded in CMakeLists.txt:

set(VODEparamfile "${CMAKE_CURRENT_FUNCTION_LIST_DIR}/integration/VODE/_parameters")

and I have to manually change it to

set(VODEparamfile "${CMAKE_CURRENT_FUNCTION_LIST_DIR}/integration/BackwardEuler/_parameters")

in order to use BackwardEuler integrator. If I don't make this change and include integration/BackwardEuler in cmake, I get complain about max_iter not defined. It would be nice to be able to choose whatever integrator you like in your code and get the right paramfile.

Context: this is part of the effort to couple multi-group radiation hydrodynamics with thermochemistry in Quokka.