population_template: Handle contrast with zero weight

[Lestropie]

I believe it had been suggested at some point by @maxpietsch that use of the CSF ODFs for registration / template construction could be detrimental. However there are circumstances in which having a three-tissue-decomposition template could be beneficial. One approach is to simply use the WM and GM ODFs for registration / template construction, save the non-linear warps, and then apply those non-linear warps to the CSF ODFS and average in template space.

An alternative approach would be to include the CSF ODFs as input to population-template, but give that contrast a weight close to zero. However this means that these data will nevertheless be included in computations by mrregister.

Ideally, population_template would catch the special case where a particular contrast is given a weight of zero. it would then:

• Omit that contrast from any mrregister calls
• If -nocleanup, still execute any transformation calls to produce intermediate template images with that contrast
• Generate the user-specified output template image for that contrast