The 'conductivity' and 'cdim' fields were temporarily added to ChBody class in the past for experiments with interactions with electrical fields, but now we are removing them because they are not beeded anymore.
(A more correct way to add custom data, if needed, would be to inherit a custom class (with custom data fields) from ChAsset and attach it to the body with the AddAsset() function )
New functions to fetch invariants from tensors
Support of custom color plot for FEM meshes
Support for plotting hydrostatic 3D stresses and strains (volumetric invariant of tensors)
Basic colour plotting for 3D beams from unit_FEM
The class hierarchy for FEM elements have been heavily changed.
In this commit, many other changes have been done to FEM code, mostly related to class interfaces.
Easier method to build a frame from a position and a rotation angle about a known unit vector
Easier method to build a ChCoordsys from a position and an angle of rotation about a unit vector
New mnemonic functions for trasforming  points, and two functions for transforming directions (not points).
There were similar functions in ChBody, now I slipped them down closer to base classes.
Note that the inheritance from ChShared has been changed from normal
   public ChShared
into
   public virtual ChShared
because there are more than one class that inherits from ChShared, and some other classes that have multiple inheritance from them, so it creates the famous 'diamond' problem of multiple inheritance.
Using 'virtual' fixes the problem forever.
 I hope this has no impact on performance because the Ref() and Unref() calls in ChShared items, called by shared pointers,  now become virtual.
MAJOR refactoring:
the new class ChBodyFrame has been inserted 'in between' ChFrame and ChBody.
This class ChBodyFrame has been used also as a base for some FEM nodes (namely, those for end-rotations of beams, i.e. nodes that have the same 6 dofs as the rigid bodies). The new class defines basic interface to simulation items with 6dofs, but it is more lightweight than the full ChBody.
The nice side effect is that one will be able to use constraints on both ChBody objects and ChNodeFEMxyzrot objects, to join beam trusses and rigid bodies, for instance using the constraints in the ChLinkMate classes.
The drawback is that lot of classes needed some refactoring.
Merge branch 'develop' into feature/dem
cleanup.