Chroma

[linahv]

Two things:

• Unexpected optional quadrupole inputs in the fit_chroma. Quadrupole indexes and a variation of their strengths could be implemented in the SC structure prior to chromaticity correction. The returned lattice would include said changes in quadrupolar strengths.
• Documentation related: added a description according to SClocoLib. Added a line in the /doc/modules/correction.srt to create a section on Chromaticity. Let me know if another outline was planned.

Function remains to be tested.

[linahv]

Input ordinates can have different lengths.

Testing successful with the principle of the fitting and optimiser. Please verify, I got an error trying to get the end chromaticity of the resulting SC instance.

[linahv]

Test passed:

• changed atlinopt calls to SC.RING.get_chrom()
• removed the two lines to set the sextupole strengths since the ring is updated within the optimiser
• Sextupole family arrays can have different lengths