MAD-X Reader: RBEND

[ax3l]

Add support for MAD-X RBEND elements in our src/python/impactx/MADXParser.py (this is used by load_file).

An RBEND will translate to an ImpactX DipEdge, SBend, DipEdge.

[cemitch99]

Note: The definitions of the parameters E1 and E2 need to be determined from the bend angle, and the correct relationship between the straight (projected) length and the arc length needs to be used.