dg : rewrite strong boundary condition for continuous galerkin

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speed-up reading post-pro files in .msh with huge number of time steps (usually sparse) - for gauthier

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compounds, transfinite, etc. easier

the coordinates are now a regular dgDofContainer. The corresponding function is obtained through dgGroupCollection::getCoordinatesFunction

- new healGeometry code from Netgen
Thanks to Ashish for the original patch

Introduce MPI in NonLinearMechSolver

(in nonlinear transient analyses, one typically saves results at regular time intervals, corresponding
to arbitrary time step numbers). 
Application of the plugin results in a view with dense step numbering, which can still cause 
difficulties when combined with another view on which this treatment has not been applied.

This reverts commit e117f3998aa016b78cc3d3431b4f5c86bb8f8b9d.