shbasis: Improved decision heuristic
shbasis: Improved decision heuristic

Improved detection of spherical harmonic basis in cases where the ratio of m!=0 to m==0 power decreases from the ideal ratio for smaller l.
This is done by using the power ratio at lmax if the gradient of the linear regression is positive.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3