spetialization of linearSystemPETSc to avoid code duplication

can control its behaviour on empty matrices

petsc : zeroMatrix() : do not call MatAssembly on empty matrix, it destroys preallocation on petsc 3.4

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    -- Add Point Mapping from ABC space to XYZ space
    -- Handle Elements with no Face

** LineBasis:
    -- Works again

** Jacobian and GroupOfJacobian:
    -- Computes Good Jacobian from Basis ReferenceSpace

** Formulation:
    -- Addapted for good Jacobian

** TermProjection:
    -- Addapted to compute function to project at good point (see ReferenceSpace ABCtoXYZ mapping)

** Laplace, Poisson, SteadyWave:
    -- Passed with all basis

** WARNING:
    -- Tet Basis NOT Retested (but seems to work)
    -- FunctionSpace::getDof doesn't work if wrong element type (because of basis implication)
    -- SystemAbstract::fixCoef doesn't work anymore (see above)
    -- Problem with cureved Element (getTypeForMSH gives HO element for FunctionSpace::getDof)

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ReferenceSpace computes Jacobians: Seems OK -- TriEdgeBasis seems to be OK -- FunctionSpaceScalar and FunctionSpaceVector adapted with new ReferenceSpace::getJacobian

- explicit declaration of (used) inherited functions is required for templatized
classes such as FaceVector
- protected openmp pragmas

  ++ Now Dofs are numbered correctly
  ++ Tri, Quad and Tet Nodal Basis looks ok
  -- Need to compute Jacobians

Basis: Interface modification