Reduce memory-usage of `vegas_adjust_grid`

- significantly reduces memory allocations; instead of two vectors
  created for each dimension now there is one (`tmp`) created that is
  reused during the grid adjustment procedure
Rename `piecewise_constant_pdf` to `vegas_pdf`
Move class vegas_point into a separate file
Change include quotation to avoid system headers
Update MPI m4 macro
Sanitize MPI handling and add test for mpi_vegas