WIP: Added CbmDigiTrigger.h, CbmAlgTriggerDigi.h and CbmAlgTriggerDigi.cxx in algo/.

Development as requested in Redmine issues #2266 and #2262.

The seed finder was reimplemented in the algo directory as CbmAlgTriggerDigi, and now derives from a base class called CbmDigiTrigger, which handles the interface to CbmDigiTimeslice.

The derived class is based on CbmSeedFinderSlidingWindow and differs only in that it adds the new interface. It still also supports an interface which accepts digi vectors directly or explicit digi time lists. This can be used by directly calling FillSeedTimes() instead of the overloaded "()" operator of the base class.

If support for the old interface should be dropped in the future, I would suggest doing this by making FillSeedTimes() private, as its existence would still make sense in the context of the algorithm logic. We will then lose the option to use explicit seed digi time lists as input.

This merge request is work in progress, as it only represents a partial solution to the Redmine issue. A full solution is not possible at present time, as CbmDigiTimeslice currently only supports STS data. Work can be completed once support for other detectors is added.

This MR includes modified CMakeLists.txt files and a CbmAlgLinkDef.h file. These are placeholders, which allow compilation of the new classes, but should be dropped once official versions are available in algo/.

@v.friese Please review.