WIP: VTKRecorder updates for recorders: more documentation and a bit less of not-so-meaningful quanitities

The various commits of this MR

• finally take care #262 by getting rid of the not-so-meaningful stress VTKRecorder.recorders (see the discussion in the issue). User scripts that would specifically ask for this recorder would run with no problem (and a warning that this recorder has been replaced with a more meaningful one) but possible Paraview post-processing could be affected (for good, in my opinion)
• remove an associated not-so-meaningful (and now unused) Shop C++ function (see the issue)
• precise the doc of some VTKRecorder-generated quantities (including the component-wise absolute value of absForceT which is still not-so-meaningful as a vector in my opinion but I stopped cleaning before erasing that one)

I have no clear opinion whether commits should really be squashed or not.