Need BoundaryFeature for user-defined outlines

[markstock]

A new BoundaryFeature subclass needs to be created for "user-defined" outlines, for which some external system generates a set of N coordinate pairs (x,y), assumed to have the first connect to the last (ultimately multiple sets of outlines and holes).

This set of pairs could come from a user-generated and selected text file, the user drawing on the screen, an airfoil file in Selig or UI-A formats, or the wall boundaries from a CFD-standard mesh file format (like from GMSH). The user would have the option to force only one computational element between each adjacent pair of nodes, or to allow Omega2D to re-discretize the outline into uniform-sized elements.

This seems to be a necessary first step before fully implementing a gmsh reader.

Important question: how would this interact with BoundarySegments?