Rename TheorySymbol to ConstantLabel. Add SchematicLabel marker for symmetry

src/main/scala/lisa/kernel/fol/CommonDefinitions.scala

src/main/scala/lisa/kernel/fol/FormulaLabelDefinitions.scala

src/main/scala/lisa/kernel/fol/TermLabelDefinitions.scala

src/main/scala/lisa/kernel/proof/RunningTheory.scala