Further improvements to shbasis
Further improvements to shbasis

The calculation of the m!=0 to m==0 power ratio for determining the SH basis, and checking of this power ratio across orders for checking quality of DW encoding scheme, have been improved.
These calculations are now highly dependent on lmax:
* If lmax=2, then that is the only order that can be used for the assessment.
* If lmax=4, then l=4 is used to determine the SH basis.
* If lmax=6, then l=4 is used to check the SH basis; the difference between l=4 and l=6 is also assessed, but the threshold for reporting a poor encoding scheme is doubled; therefore there needs to be a very large difference in the power ratio between these two orders for it to be reported to the user.
* If lmax>=8, then a linear regression is performed between l=4 and l=lmax. The value of this regression at l=0 is then used for determining the SH basis (this is more reliable when either the DW encoding is poor, or for FOD images affected by the CSD norm regularisation bug), and the gradient of the regression is used to report on the DW encoding quality.

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