Be able to specify the lattice element attribute to be modified in order to adjust chromaticity #140
Description
When adjusting chromaticity, sometimes the element is a sextupole with attribute k2 but sometimes it is another kind of element like a multipole with attribute k2n or something else. Besides marking the elements to be used as correctors, we should be able to specify what attribute to change to adjust the chromaticity which could be possibly different between horizontal and vertical correctors.