Restructure information exchange for integrators:
- integrators receove struct pointer instead of variables
- include information that is needed for PRD calculation
PRD implementation:
- Calculate particle-constant part of PRD in separate function
- Main calculation in calc_accel
- Print out particle radius and add it to the configuration_values struct
- Add CMake option for PRD
Merge branch 'md_PRD' into GSOC-Integrator