Feature/make binary

Add function: make_binary for creating binary rasters. E.g.: find all wetland in a land cover raster.

Branch: https://gitlab.com/robit.a/wildcam/tree/feature/make_binary

• sufficient tests
• complete man
• examples
• add to intro vignette
• update NEWS
• bump vers
• tag release
• fix bug #18 (closed)
• non equi, gte, lte, %in% methods with fun argument