Fix behavior of SACMODEL_PARALLEL_LINE

This pull request fixes the behavior of `SACMODEL_PARALLEL_LINE` and makes some clean up in the related files.

Despite the name and in contradiction with the documentation, currently the model searches for lines _perpendicular_ to the given axis. This issue has been brought up on the [mailing list](http://www.pcl-users.org/SACMODEL-PARALLEL-LINE-parallel-or-perpendicular-to-axis-td4000669.html) a long while ago, but no action was taken at that time.

The additional commits in this pull request:
  * remove trailing spaces;
  * fix documentation;
  * put instantiations of `SACMODEL_LINE` and `SACMODEL_PARALLEL_LINE` in separate `cpp` files;
  * add `const` qualifiers where appropriate;
  * add a unit test for `SACMODEL_PARALLEL_LINE` with the new, correct behavior.