ENH: generic/arbitrary input format for VBM

[raamana]

It would be useful for users to offer a toolbox-independent format specify volumetric voxelwise probabilistic images. Two possible options are

1. folder of folders with a fixed file name (prob.nii?) for image such as
   --input_dir /project/vbm --id_list idlist.txt where /project/vbm would contain a bunch of folders each referring to one of the subject IDs in the id_list which must contain prob.nii
2. a single CSV file with two columns, the first being the subject ID and the second being full path to the corresponding image (no relation between ID and path enforced):

id1,/project/vbm/id1/somename.nii
random2,/project/vbm/notnecessarily_random2/someothername.nii
arbitrary3,/project/vbm/whateveruserchooses/filename.nii

Choose one and implement it.