Now the grid refers to the centroid grid

Still working on double indexes
Interpolation fixed
Finally the interpolation works correctly

Included everything into a function
Small amendments.

The program is now complete up to the point of collecting all the
velocity data from the StarCCM simulation
Fixed interpolation

This version fixed the interpolation of velocities
I have also added two functions. One simply adds a wind rose to the
velcoty field
The returns the velocity field for each wind speed and direction