Add population template step

[ebrahimebrahim]

Add population template step.

• We first need to have chosen a registration method.
• The input should be a set of images, of whatever modality the registration method wants.
• The output should be a set of warps to a common space. The warps will be applied to various scalar images (FA for generating a white matter skeleton for TBSS, and NODDI images for projection to the skeleton). So the format of the warps is up to us -- it should be whatever is most convenient for later applying the warps to scalar images.

Before this task begins, we need: A registration method, either something we create here or something else. There could be a placeholder "basic" method that doesn't need GPU.

[ebrahimebrahim]

Follow this dtitk population template builder to help create a generic one or one that uses whatever our preferred registration method is.