Revise some qualifiers of the function 'orbit_calculator'

- The qualifiers 'r_disk' and 'N_disk_crossings' are lumped together
  into a qualifier called 'disk', which is a structure that allows to
  define more generic disk geometries.
- The default value of the qualifier 'MC_runs' is changed from 10^4
  to 10^5.
- The default value of the qualifier 'tolerance' is changed from
  10^-10 to 10^-8.