Initial tweaks to GMWMI seeder

Once the GM-WM interface is found by the seeder, the position is perturbed by up to 2 voxels along the plane defined by the local normal, and the position is re-optimised.
GMWMI seeding: Change to floating-point image

Command 5tt2gmwmi will now generate a floating-point image for GMWMI seeding, rather than a mask.
Internally, the GMWMI seeder then uses the rejection seeder to determine the initial seed points before interface optimisation.
Using tri-linear interpolation in the rejection seeder prior to seed point selection, rather than after, was implemented, but found to not have a significant effect in this circumstance, so is not used by default (as it will otherwise result in seeds in voxels with zero intensity adjacent to non-zero intensity voxels)
Bug fix in GMWMI seeder

Error was resulting in unnecessary rejection of many seeds (accepted by the seeder but failing the ACT test of the seed), which was resulting in inhomogeneity in the seed distribution.
Merge branch 'gmwmi_perturb'