BUG: initial_moving_transform_com needs default

The initial_moving_transform_com needs to be defaulted
to false in order to maintain backwards compatibility.

When this is not default = False, it results in

initial_moving_transform = <undefined>
initial_moving_transform_com = <undefined>

Error:
Registration requires a value for input 'invert_initial_moving_transform' because one of initial_moving_transform is set. For a list of required inputs, see Registration.help()