Feature/chilgenb conditional cmake

[chilge]

Make sbndcode and icaruscode both optional dependencies but require that CMake finds one active instance (following the mrbsetenv step). In order to avoid clashing dependencies, do not permit both icaruscode and sbndcode to be active simultaneously.

[vitodb]

@chilge in the code for this PR, it looks like we are allowing the case where both experiment codes are setup in the environment. Maybe we want avoid this case as we could get some library clash. What do you think?

[chilge]

@vitodb I added a check in the top-level CMake file that will throw a fatal error if both icaruscode and sbndcode are active at build time.