undulators: add 'undulator_prefix' in configuration

Scientists like to call undulators with a different name, so instead of defining undulators like this:

name: u27d
attribute_position: U27c_GAP_Position         # see the u27d->u27c translation
attribute_velocity: U27c_GAP_Velocity         # repeated for velocity...
attribute_acceleration: U27c_GAP_Acceleration # repeated for acceleration...
# missing FirstVelocity !
steps_per_unit: 1
velocity: 5
acceleration: 45
low_limit: 10
high_limit: 300

I would prefer this:

name: u27d
undulator_prefix: u27c_gap
...

We can deduce attributes automatically, without repeating the different attributes since the prefix is the same.