Dedicated class for polynomial ramps of arbitrary order!

Cleanup and documentation!
ramp: New class for polynomial ramps. Instantiated in the ForwardModel.
forwardmodel: Now uses new ramp class.
phasemapper: Ramp functionality moved to ramp class.
reconstruction: Ramp order not used, ramp can be now retrieved from the ramp
                class after reconstruction.
analytic, magcreator: Better and consistent definition of pixel centers and
                      their influence on the phase.
testing: Minor corrections for new syntax.