
* add equal order approximation terms
  x GaLS or IP

* add convection stabilisation terms
  x GaLS or IP


* add divergence jumps stabilisation terms
  x IP

* externalize the time loop
  x provide a function that does a NS time loop given the time and a
   time step
  x allows to treat multi-physics

* implement optimal quadrature formulas; the cost is really too high
   in 3D

* restart strategy in BDF
  BDF( Tinitial, Tfinal, dt, BDF_STRATEGY )
  enum BDF_STRATEGY { BDF_STRATEGY_DT_CONSTANT,
   BDF_STRATEGY_DT_ADATIVE };


- save results in subdirectory bdf_o_<oder>_dt_<dt>/ while doing
   shiftRight()
- use a master file to save meta data (order, Tinitial, Tfinal, dt,
   timesteps differences saved...)


* impose weakly pressure to 0 at outflow