Add flag to choose overwrite, skip or append

[lbogaardt]

When LaserChicken extracts features from an input pointcloud, it creates an outplut .ply file. When new features are extracted from the same input, the output file is currently overwritten (I think). Perhaps add a flag so the user can determine whether:

1. All features should be recomputed and the entire output file overwritten
2. Nothing should be computed, the file is skipped
3. Only new features should be computed and appended to the existing .ply file.

In either case, the documentation could be made more clear about the current behaviour of LaserChicken (which is option 1., I think).