Add vm2

[RomanIlchenko1308]

This PR adds VM2.

[RomanIlchenko1308]

Partial implementation of changes suggested in code review.

[akaszynski]

Looks like we're getting an error when evaluating:

a = mapdl.post_processing.plot_nodal_eqv_stress()

~/ansys/source/pymapdl-reader/ansys/mapdl/reader/mesh.py in _surf(self)
     65         """External surface"""
     66         if self._surf_cache is None:
---> 67             self._surf_cache = self._grid.extract_surface()
     68         return self._surf_cache
     69 

~/ansys/source/pymapdl/ansys/mapdl/core/mesh_grpc.py in _grid(self)
    462         if self._grid_cache is None:
    463             self._update_cache()
--> 464             self._grid_cache = self._parse_vtk(force_linear=True)
    465         return self._grid_cache
    466 

~/ansys/source/pymapdl-reader/ansys/mapdl/reader/mesh.py in _parse_vtk(self, allowable_types, force_linear, null_unallowed, fix_midside, additional_checking)
    158 
    159         # Store original ANSYS element and node information
--> 160         grid.point_data['ansys_node_num'] = nnum
    161         grid.cell_data['ansys_elem_num'] = self.enum
    162         grid.cell_data['ansys_real_constant'] = self.elem_real_constant

~/.local/lib/python3.8/site-packages/pyvista/core/datasetattributes.py in __setitem__(self, key, value)
    216 
    217         has_arr = key in self
--> 218         self.set_array(value, name=key)
    219 
    220         # do not make array active if it already exists.  This covers

~/.local/lib/python3.8/site-packages/pyvista/core/datasetattributes.py in set_array(self, data, name, deep_copy)
    595 
    596         """
--> 597         vtk_arr = self._prepare_array(data, name, deep_copy)
    598         self.VTKObject.AddArray(vtk_arr)
    599         self.VTKObject.Modified()

~/.local/lib/python3.8/site-packages/pyvista/core/datasetattributes.py in _prepare_array(self, data, name, deep_copy)
    749             data = tmparray
    750         if data.shape[0] != array_len:
--> 751             raise ValueError(f'data length of ({data.shape[0]}) != required length ({array_len})')
    752 
    753         if data.dtype == np.bool_:

ValueError: data length of (10) != required length (17)

[akaszynski]

We're now tracking this in #736

[akaszynski]

See suggestions. Most notable is using get_value instead of get.

Additionally, use nplot or kplot to visualize the engineering problem from MAPDL whenever possible.

[akaszynski]

Made several changes to clean up the style a bit. Note how I added a sphinx cross reference to the function:

:py:meth:`Mapdl.get_value <ansys.mapdl.core.Mapdl.get_value>`.

See:
https://stackoverflow.com/questions/22700606/how-would-i-cross-reference-a-function-generated-by-autodoc-in-sphinx

You may want to also consider building the documentation locally. I think pycharm does a good job rendering the example rst, but there are some idiosyncrasies to our doc build that may not be rendered in sphinx.

Final suggestion:
Would it be possible to work out the hand-calculations for this? I'm fine with not having them, but if you've already worked out the hand calculations, you can just include it in the analytical section at the beginning of the example.