Refactor spherical mean estimation of data

[rutgerfick]

Current implementation in dmipy/dmipy/utils/spherical_mean.py is very slow and ugly with shell-wise for-loops. In spherical mean model fitting it is often half the estimation time.

• precalculate and pre-invert multishell spherical mean observation matrix A_sm_inv in acquisition scheme
• multi-shell spherical mean estimation should be nothing more than sm = np.dot(A_sm_inv, data)