Better handling of output layers

[AlisterH]

We should either:

• generate output grids as memory layers (probably less work, and better because the user might want to generate several grids and choose to keep the best one), or
• improve handling of output layers:
  • remember the previous output directory
  • automatically create the output directory if it doesn't exist
  • allow to overwrite existing files

[AlisterH]

allow to overwrite existing files

Actually, this works as long as there is no file lock (so you need to remove the layers from QGIS first).