Parametric insert pipe bend

Creates an insert pipe bend that sits flush with a pipe of the specified size, yielding a smooth outer surface when assembled.

Usage:
insert_pipe_bend(bend_angle, bend_radius, insert_length, pipe_outer_diameter, pipe_inner_diameter );

The resulting bend has an angle of bend_angle, with a radius *measured along the centerline* of bend_radius. At each end is an insert section of length insert_length to slide into the connecting pipes. The connecting pipe dimensions are specified by the