Fix MlsSphereFitDer weight derivatives (issue #95)

[ThibaultLejemble]

Here is a quick fix to the issue #95.
The basis center is added to the local point to get back the point in global coordinates, and then call weight derivatives functions.

Small disadvantage: the basis center is added here, but then subtracted in the weight function, so it is not the most efficient solution (except if the compiler is good enough to avoid these successive additions and substractions).

[nmellado]

Actually the bug can be simply fixed by accessing the global coordinates with attributes.pos().
The documentation should be also fixed.