[cleanup] use delegating constructors for AffineGeometry
Since we started to require C++11-compatible compilers, we can now make use of delegating constructors to remove code duplication and make the intention of the constructors more clear.
AffineGeometry's constructors always come in pairs: one taking a ReferenceElement and one taking a GeometryType. Since the GeometryType is only used to look up a ReferenceElement and then proceed identical to the former constructor, we can instead delegate the actual construction from the latter to the former constructor.