[doc][doxygen] add IO group

doc/doxygen/modules.txt

@@ -43,3 +43,8 @@
      * \defgroup Intersection Intersection
      * \brief Classes and functions related to intersections between geometries.
      */
+
+/*!
+ * \defgroup IO Input-Output
+ * \brief Classes and functions for file input/output.
+ */

frackit/io/brepwriter.hh

@@ -18,6 +18,7 @@
  *****************************************************************************/
 /*!
  * \file
+ * \ingroup IO
  * \brief Class that writes entity networks into the .brep file format.
  */
 #ifndef FRACKIT_BREP_WRITER_HH
@@ -46,6 +47,7 @@
 namespace Frackit {
 
 /*!
+ * \ingroup IO
  * \brief Class that writes entity networks into the .brep file format.
  *        This creates a single TopoDS_Compound shape in which each sub-shape
  *        is uniquely defined.

frackit/io/gmshwriter.hh

@@ -18,6 +18,7 @@
  *****************************************************************************/
 /*!
  * \file
+ * \ingroup IO
  * \brief Class that writes entity networks into Gmsh .geo file format.
  *        Effectively, this will write a .brep file and a .geo file in
  *        which the .brep file is included. Then, physical definitions
@@ -34,6 +35,7 @@
 namespace Frackit {
 
 /*!
+ * \ingroup IO
  * \brief Class that writes entity networks into Gmsh .geo file format.
  *        Effectively, this will write a .brep file and a .geo file in
  *        which the .brep file is included. Then, physical definitions