extended support for maps, added the standard map example, fill command

- guessing for maps is now working with InternalMapStepper
- inverse maps can now be specified
- fixed two bugs in Wave::process (bufferCapacity not reset + GL_WRITE_ONLY)
- x = 1 will set the variable x now, instead of the axis label to "= 1"
- added the fill command, creates new points from existing points
  with interpolation