Add option to collapse the Jones chain inside the solver

[JSKenyon]

This PR adds functionality to optionally collapse the Jones chain prior to solution. This essentially means that the Jones chain is never more than three terms long from the perspective of the solver as any longer chain can always be reduced to at most three terms (and occasionally two). This should speed up calibration in cases where the chain becomes very long e.g. 1GC with polarisation, or where there is a direction-dependent term in the chain (which are expensive to apply on-the-fly). This functionality is enabled by default but can disabled using solver.collapse_chain=0.