Cleaning warnings

Since in a personal project that is using MRPT I'm using continuous integration with very restrictive compilation flags, I cleaned most of the warnings reported by clang compiler in the MRPT project. I left a brief summary of the changes made:

- Using default clause for working with not handed switch cases in features_matching app.
- Fixed unused-argument warnings.
- register storage class is deprecated in c++11. I removed only those not important. For example, I left the register word in SSE code related with vision stuff.
- Hidden a warning related with an unused function in the case of compiling the project with clang.
- Rearranged some variable members in constructor initialization lists for avoiding warnings.
- Using std::abd instead of C version of abs for not truncating long int to int.