Record cause of voxel dropout

[TomMaullin]

The code should also output a blmm_vox_dropout map which encodes what has caused voxels to drop out of an analysis. For example, if a voxel has;

• value 1: It was removed by masks a user specified.
• value 2: It was removed by a threshold the user specified.
• value 3: It was removed because the design was too low rank.
• value 4: It was removed as parameter estimation reached the maximum iteration limit.
• value 0: It was not removed.