- Modified computeField to use a xEntityToEntity, useful in non-symetric case.
Modified other classes and functions accordingly

- Use CustomValueDoubleOldAndCurrent to store both previous and current displacement field

- Added computation of energy release rate, with computation of crack speed
based on opening integrals. May still oscillate and depend on k

- Added a debug_values vector, which my be useful to export scalar quantities other time for debug, without needing to properly add it (like load factor for instance). Just need
to push it back into debug_values, then add "debug_0, debug 1, etc" in info.dat.