Issue #24 openvoronoi offset node crashing

- Modified the instructions to compile openvoronoi with Debug option turned off in order to disable the assert statements in that library which were causing the node to crash.  It will now return a results even if it gets an ill formed mesh/polygon boundary instead of aborting the application.
- Clears meshes_map in the surface server before new meshes are generated.  Previously, old data was kept in this map and as a result, old surfaces were being sent to the process path generator.
- This PR also addresses issue #31