Document and greatly improve TSP algorithms (#386)

- Add LEMON headers.
- Add Doxygen doc for all classes and their members.
- Clarify and unify the public API of the algorithms.
- Various small improvements in the implementations to make
them clearer and faster.
- Avoid using adaptors in ChristofidesTsp.

SET(LEMON_VERSION "@PACKAGE_VERSION@" CACHE STRING "LEMON version string.")