message: Add link to OFF file doc
diff: @@ -10,7 +10,7 @@ where `dim` is the dimension of the homological feature, `b` and `d` are respect
  **Usage**
  `alpha_complex_3d_persistence [options] <input OFF file>`
  where
  -`<input OFF file>` is the path to the input point cloud in OFF format.
  +`<input OFF file>` is the path to the input point cloud in [nOFF ASCII format](http://www.geomview.org/docs/html/OFF.html).
  **Allowed options**
  @@ -55,7 +55,7 @@ Same as `alpha_complex_3d_persistence`, but using weighted points.
  **Usage**
  `weighted_alpha_complex_3d_persistence [options] <input OFF file> <weights input file>`
  where
  -`<input OFF file>` is the path to the input point cloud in OFF format.
  +`<input OFF file>` is the path to the input point cloud in [nOFF ASCII format](http://www.geomview.org/docs/html/OFF.html).
  `<input weights file>` is the path to the file containing the weights of the points (one value per line).
  **Allowed options**
  @@ -85,11 +85,12 @@ and [Regular triangulation](https://doc.cgal.org/latest/Triangulation_3/index.ht
  ## `periodic_alpha_complex_3d_persistence` ##
  Same as `alpha_complex_3d_persistence`, but using periodic alpha shape 3d.
  +Refer to the [CGAL's 3D Periodic Triangulations User Manual](https://doc.cgal.org/latest/Periodic_3_triangulation_3/index.html) for more details.
  **Usage**
  `periodic_alpha_complex_3d_persistence [options] <input OFF file> <cuboid file>`
  where
  -`<input OFF file>` is the path to the input point cloud in OFF format.
  +`<input OFF file>` is the path to the input point cloud in [nOFF ASCII format](http://www.geomview.org/docs/html/OFF.html).
  `<cuboid file>` is the path to the file describing the periodic domain. It must be in the format described [here](http://gudhi.gforge.inria.fr/doc/latest/fileformats.html#FileFormatsIsoCuboid).
  **Allowed options**
  @@ -142,7 +143,7 @@ where `dim` is the dimension of the homological feature, `b` and `d` are respect
  **Usage**
  `alpha_complex_persistence [options] <input OFF file>`
  where
  -`<input OFF file>` is the path to the input point cloud in OFF format.
  +`<input OFF file>` is the path to the input point cloud in [nOFF ASCII format](http://www.geomview.org/docs/html/OFF.html).
  **Allowed options**
  @@ -173,5 +174,4 @@ Betti numbers[2] = 1
  ```
  N.B.:
  -* `alpha_complex_persistence` only accepts OFF files in dimension d.
  * Filtration values are alpha square values.
  