Add cuba refinement option and global options

- moved 'mpi_single_generator' into new file 'global_configuration.hpp'
Change interface of MC helper functions

- the '0' variant uses the same algorithm for computing the cumulative
  result
- the '1' variant uses simple averaging and discards the error of each
  individual iteration
- 'hep::chi_square_dof' remains but requires an additional parameter,
  the cumulative result