Replace spkezp_c with spkezr_c in RK76:
- returns position and speed vectors of body
- requires slightly different parameter formatting
Basic 5th order implementation
- only partly optimized
- very slow
Memory allocation improvements
- allocate memory for body state coefficients only once in RK76
- one code across interpolation orders for alloc/free
- allocate/free memory for body_c in separate functions
- add interp_order to config_data struct
-> very significant speed improvement.
dtime power computation cleanup:
- move precomputed dtime powers to struct
- compute only once per substep
Merge branch 'md_interpol' into GSOC-Integrator