transform_from_displacement_field segfaults

[rueberger]

the following cause segfaults:

ants.transform_from_displacement_field(ants.from_numpy(np.random.randn(200, 200, 200)))

ants.transform_from_displacement_field(ants.from_numpy(np.random.randn(200, 200, 200, 3)))

[stnava]

incorrect usage. requires multi-channel input.

    field : ANTsImage
        deformation field as multi-channel ANTsImage

could add a check for this type of bad input or maybe do a conversion internally.

[rueberger]

right, I'm aware of the correct usage, just reporting the segfault