Add missing centerOfRotation field in the xml registry for viewpoint.

Include the centerOfRotation in the viepoint transformation calculation.
Fix bounding sphere calculation for elevation grid.

The calculation of max_x was x_spacing * x_dimension, while
it is supposed to be x_spacing * (x_dimension-1) according to the grid definition.
Same for z dimension.
Reimplement bounding sphere extension by point and by other sphere.

1. There was a bug in sphere by sphere extdend when cocentric sphere would not extend current
event if with larger radius.
2. I have took this as a chance to reimplement extend methods using vector arithmentic. This
may be less efficient from number of floating point operations point of view. But the code is much
more readable now.