tbt optics correction

[elafmusa]

To do:
-apply_correction function to be replace by SC.set_magnet_setpoints
-Coupling RDTs correction to be added

[SebastienJoly]

I have rewritten the functions in tbt.py. I tried to remove some redundancy and take advantage of pySC/pyAT functions whenever possible.
Please remember that I now compute the phase advance from the BPM readings and NAFF algorithm. Therefore, the corrections are less effective than with the function at.get_optics but also more realistic. Of course, this is only a first draft from my side, we can still further improve the module!
As the next steps, it would be nice to be able to choose which observables to feed to the function (e.g. phase beating, tune, dispersion, etc) in a generic way. It would allow us to reuse it effortlessly to correct coupling RDTs or other quantities.