Refactor grid code

- a VEGAS grid is now no longer a std::vector<std::vector<T>> but
  objects the class linear_grid<T> instead
- linear grid may be used with the iostream classes, e.g. to save/load a
  grid. The example 'examples/read_linear_grid' shows how to do this;
  'examples/grid_file' is an example grid file
Fix a small problem in test_plain.cpp