Update MultipoleTStraight default expansion order parameter

The parameter that controls the expansion order in MultipoleTStraight currently defaults to 0 (MAXFORDER). This yields poorly conditioned fields in the fringe field with bad divergence and curl very non-zero for quadrupoles and dipoles. For higher order multipoles, it yields body fields that are entirely incorrect.

The MAXFORDER parameter should default to a reasonable number, so that higher order multipoles are rendered correctly by default with a couple of orders of higher non-linear terms in the fringe field.