Add higher order momentum compaction factor

Add mcf_order optional parameter for Synchrotron class to prodivde higher-orders momentum compaction factor.
Add Synchrotron.get_mcf_order method to compute momentum compaction factor up to 3rd order from AT lattice.
Modify Synchrotron.eta to use higher order mcf.
Improve Synchrotron docstrings.
Change LongitudinalMap to use Synchrotron.eta.