v0.2.2

• added two examples showing the advanced use of the VEGAS integration routine
• added a callback mechanism for the VEGAS routines; this enables one to print
  intermediate results from completed iterations
• added VEGAS routines that reuse existing grids
• grids can now be easily saved to files with iostream shift operators, see the
  example examples/read_linear_grid.cpp which can be used with the grid file
  in examples/grid_file