NEW : GetCurrentMap()

- This method is intended to return the VectorField Pointer m_ptrMapIn to be able to evaluate the displacement during the registration process.
NEW : GetDisplacement() in VectorField object

- method that returns the mean value of the norm of each element in the vectorField
- If the element is dimension 2 or 3, sqrt() is used for each element to get the vector norm : this cost a lot!
NEW : Using displacement as a stopping condition

- We use GetCurrentMap to get the vectorField that caracterize the displacement
- We subtract to this vectorField the previous vectorFIeld, cellwise, to get the difference in displacement since the last iteration
- We use the method GetDisplacement to get aq scalar value from the vectorField to evaluate for the stopping condition
- Here, we stop under 0.5, which I assume means if the displacement is less than half a pixel ?