FINITE DIFFERENCE WAVEFRONT ESTIMATE
shackHartmann: finiteDifferenceWavefront property now provides a wavefront
 reconstructed using the finite difference sparse matrix computed with the
method sparseGradientMatrix
SHACKHARTMANN FIXES:
in relay: the default is now the WFS point towards the source by setting
the initial value of pointingDirection to empty
in get.finiteDifferenceWavefront: for multilple sources
NEW HEAD
NEW HEAD