Clean-up of mrpt-vision API

[jlblancoc]

To-do list:

• [x] Upgrade IplImage -> cv::Mat (See #710)
• [x] Re-think features tracker, if needed.
• [x] Remove un-needed classes.
• [x] Improve docs in lib doxygen page.
• [x] Go thru all the MRPT_TODO's in this library and resolve them.
• [x] Delete all old SIFT implementations
• [x] Remove CFeatureList

[jlblancoc]

Update on this one: I think CFeatureList should be entirely removed, leaving only TSimpleFeatureList for feature points, and a new alternative to CFeatureList for storing descriptors.

Part of this was done in #889 , but the alternative to CFeatureList is left as future work since it's not urgent for me right now.

Note that CFeatureList is used in some past-years GSoC GUI apps, so refactoring would imply some amount of work.

IMO, this ticket would be a good GSoC 2019 project, BTW...