change how local_connect handles NEB parameters

The parameter tree is now changed.

params.double_ended_connect.local_connect_params.NEB_image_density
params.double_ended_connect.local_connect_params.NEB_max_images
params.double_ended_connect.local_connect_params.NEB_iter_density

are now accessed through

params.double_ended_connect.local_connect_params.NEBparams.image_density
params.double_ended_connect.local_connect_params.NEBparams.max_images
params.double_ended_connect.local_connect_params.NEBparams.iter_density

note: the dictionary NEBparams now cannot be passed directly to the
class NEB. It will give an error because NEB doesn't accept those
parameters.  It is now meant to be passed to a wrapper like create_NEB