Add new NDVI filter

Ok. I cleaned up the history a little. This pull request now consists of two commits:

1) Change the frei0r.hpp string behavior: https://github.com/pez4brian/frei0r/commit/1e422b43e3a2ad418d2d6080ad8f80192726c98d
and
2) Add the NDVI filter: https://github.com/pez4brian/frei0r/commit/7fd7bb128a00d2c9d3c86d8d52ad913281b0a17e

If you merge this pull request, the new history will show these two, distinct commits.