new command sh2response to estimate response from SH coefficients

This does nothing more than convert SH coefficients in the voxels
specified into response coefficients, assuming the fibres are aligned
with the directions provided. It is up to the user to provide images for
the SH coefficients, the single-fibre mask, and the fibre directions.
This provides complete flexibility in how each of these is computed.

For instance, this allows SH coefficients estimated using the -rician
option of amp2sh to be used to provide relatively robust rician bias
correction. It also allows non-standard ways of computing a single-fibre
mask (whether through recursive calibration, FA mask, etc.), and also of
estimating the fibre orientation (e.g. tensor eigenvector, CSD peak
direction, segmented fibre lobe mean direction, ...). Hopefully this
will prove useful to build custom scripts for e.g. multi-shell response
function estimation, etc.