Test betatron line

[lgiacome]

To test that the conventions in the wakes are correctly implemented the script in example/gr_resonator_sps.py should give an instability like in the picture.

The test checks that when a very narrow resonator tuned on a destabilizing betatron line is added, an instability takes place.

If there is no instability one can check if the instability is seen again when setting fline = (9240*f_rev + (0.13)*f_rev) with the plus sign. If the instability is seen again it probably means that a wrong minus sign was introduced in the wake.