Model outlier removal contribution

Is it the good way to update @TimoHackel's contribution #254 ?

 - [x] Rebase on current master
 - [x] Licence update

- Formatting
 - [x] Update with Eclipse formatter
 - [x] Correct weird line breaks
 - [x] Correct member field names format (add  `_` suffix)

- `single_threshold` method:
  - [x] Rename to `checkSingleThreshold`
  - [x] Improve (reduce) source code

- Others
  - [x] Use `SacModel` enum instead of `int`
  - [x] Fix `getThreshhold` typo
  - [x] Mark getter functions with `const` modifier
  - [x] Remove `model_from_normals_` class member
  - [x] Remove `PCLPointCloud2` specialization
Thanks for the reference, it helped me. I think what I have done is correct!