Fix bug causing integer-overflow on 32bit arches

- on 32bits systems the maximum number that can be squared with unsigned
  32-bit integers is sqrt(2^32-1) which is aprroximately 2^16 = 65536.
  This was the maximum number of function calls for VEGAS. If this limit
  was supassed the error(s) were NaN.
Reuse the 'plain' function in 'mpi_plain'
Print percentage error in 'vegas_verbose_callback'
Introduce return value for callback functions

- if the return value is 'false' all remaining iterations are skipped
- this introduces a backwards incomptabile change - the callback
  function now has to return a bool instead of nothing (void)
- added example that illustrates how to use this feature to stop after a
  certain precision is reached