Detect extreme cases at contours to surface conversion

[cpinter]

..and indicate failure instead of unreasonably long computation time or possibly crash. The extreme cases include

• Special cases for most of a structure, e.g. 90% or more keyholes or branching contours

Migrated from https://app.assembla.com/spaces/slicerrt/tickets/789-detect-extreme-cases-at-contours-to-surface-conversion/details