The file opt_control/target.F90 is extremely long to compile

When recompiling with cmake, this file always takes very long to compile. This probably indicates too many dependencies and a too complex code. Maybe this could be restructured such that it is i) compiled less often, and ii) compiles faster.

In general, we should try to identify files that slows down the compilation, to make developers' life easier.

@martin.lueders @AlexBuccheri Any volunteer for this?