Add a callback mechanism for VEGAS

- vegas_callback can be used to set a function that is called after each
  iteration in vegas/mpi_vegas, e.g. to print the results as soon as
  they are available
- added two standard callback functions:
  - vegas_default_callback that does nothing and
  - vegas_verbose_callback that prints everything to stdout
- added the same functions also for the MPI VEGAS, with prefix 'mpi_'
Use callback function in examples
Fix small bug that prevented `make dist`