SNES petsc interface - user function for building the Jacobian (f'(x)) and the...

SNES petsc interface - user function for building the Jacobian (f'(x)) and the Residual (-f(x) = b-Ax)
NL loop governed by PetSC in this case ==> Use with Generate/GenerateJac + SolveNL
Useful SNES options for petsc: 
-snes_monitor
-snes_converged_reason
-snes_type ls/tr
Check on Petsc manual for more!

Some cleaning up in MovingBand2D routines...