/*!

\page split_to_tetra_page Splitting volumes

\n This operation allows to split either any volumic elements into
tetrahedra or hexahedra into prisms. 2D mesh is modified accordingly.

<em>To split volumes:</em>
<ol>
<li>Display a mesh, a sub-mesh or a group in the 3D viewer.</li>
<li>In the \b Modification menu select the <b>Split Volumes</b> item or
click <em>"Split Volumes"</em> button in the toolbar.

\image html split_into_tetra_icon.png
<center><em>"Split Volumes" button</em></center>

The following dialog box will appear:

\image html split_into_tetra.png

<br>
<b>Target element type</b> group of radio-buttons allows to select
a type of operation. If \b Tetrahedron button is checked, then the
operation will split volumes of any type into tetrahedra.
If \b Prism button is checked, then the operation will split hexahedra
into prisms, and the dialog will look as follows:

\image html split_into_prisms.png

<ul>
<li>The main list contains list of volumes to split. You can click on
  a volume in the 3D viewer and it will be highlighted (lock Shift
  keyboard button to select several volumes). Click \b Add button and
  the ID of this volume will be added to the list. To remove the
  selected element or elements from the list click \b Remove button. <b>Sort
    list</b> button allows to sort the list of IDs. \b Filter button allows to
  apply a definite filter to the selection of volumes.
  <br><b>Note:</b> If you split not all adjacent non-tetrahedral
  volumes, your mesh becomes non-conform.</li> 
<li><b>Apply to all</b> radio button allows to split all
  volumes of the currently selected mesh.</li>
</ul>

<ul>
<li><b> Split hexahedron </b> group allows to specify a method of
  splitting hexahedra.

<ul>
<li><b>Into N tetrahedra/prisms</b> allows to specify the number of
  tetrahedra or prisms a hexahedron will be split into. If the
  specified method does not allow to get a conform mesh, a generic
  solution is applied: an additional node is created at the gravity
  center of a hexahedron, serving an apex of tetrahedra, all
  quadrangle sides of the hexahedron are split into two triangles each
  serving a base of a new tetrahedron.</li> 
<li> <b> Facet to split </b> group allows to specify a side (facet) of a
  hexahedron to split into triangles when splitting into prisms.
  The facet to split is defined by specifying a point and a direction
  close to normal of the facet. The operation finds a hexahedron most
  close to the specified point and splits a facet whose normal is most
  close to the specified direction. Then the splitting is propagated
  from that hexahedron to all adjacent hexahedra.
  <ul> 
    <li> <b> Hexa location </b> allows to specify a <em> start
        point </em> by which a first split hexahedron is found. <em>
        Selection button</em> switches to selection of the element whose
      barycenter will be used the start point and whose direction will be
      used as a normal to facet to split into triangles. To return to
      selection of volumes to split it is necessary to switch this button
      off. </li>
    <li> <b> Facet normal </b> allows to specify a direction of the
    normal to hexahedron facet to split into triangles.</li>
  </ul>
<li><b> All domains </b> - if it is off the operation stops as all
  hehexedra adjacent to the start hexahedron are split into
  prisms. Else the operation tries to continue splitting starting from
  another hexahedron closest to the <b> Hexa location</b>. </li>
</li>
</ul>

<li><b>Select from</b> a set of fields allows to choose a sub-mesh or an
  existing group whose elements will be added to the list as you ckick
  \b Add button.</li>
</ul>

<li>Click the \b Apply or <b>Apply and Close</b> button to confirm the operation.</li>
</ol>
*/