implement --registration_to_common_space

[FrancoisPgm]

Implement the registration of template0 to the atlas provided with --registration_to_common_space, register jacobians to that space (not tested for 2level)

Also add an error message if less than 2 inputs

[gdevenyi]

Thanks for the contributions thus far, this is great work. A few general things:

• please run pep8/pycodestyle and fix up any warnings for your added code
• please remove the "####" blocks around the newly added code
• please update the help in the argparse saying this feature isn't implemented :)

[gdevenyi]

Thanks!