Updated representation of geometric-features.
Documenting geometric features.
Added definition of feature relations.
Some corrected doc-string.
Renamed slot 'name' of feature to 'id'.
Bug-fix: corrected a type declare.
Added classes 'feature-constraint and 'feature-constraint-state.
Making sure geometric features have correct types.
Suppress warnings when creating default geometric features.
Checking/enforcing that feature relations have valid function types.
Correcting some doc-string.
Bug-fix: 'function' is a locked symbol in SBCL. Using fucntion-type instead.
Export all relevant symbols.