Updating getDisplacement()

BUG - replace x_value with fabs(x_value) in dimension 1
NEW - use max displacement instead of mean displacement
NEW - return -1 if not good dimension
Updating Displacement computation

Last commit of this file was not working : we did not store the vectorfields at intended (because a vectorfield object, being a vectorarray, does not store the data but a pointer on the data). We therefore allocate memory for two vectorfields at a time that we will be able to substract to get the displacement map. The displacement computation now works and can be used as a stopping condition.
Updating GetCurrentMap() const