Corridor Geometry

A corridor is primarily defined by a set of positions and a width.  The width of the corridor is centered on the set of positions.  Height and extruded height can also be specified.

There are three options for corner styles- round, mitered and beveled.  These can be configured with `options.roundCorners` and `options.beveledCorners`.  The default is for the corners to be mitered.  (Should I make an enum to handle this more clearly?)

I decided to do the outline of the corridor in a separate pull request.