get_boundaries already needs labeled landscape

[mhesselbarth]

get_boundaries needs an already labeled landscape to return the correct result. I think it would be more consistent with other functions if you could pass the whole landscape to get_boundaries with an option which boundaries you want similar to e.g. show_patches

[mhesselbarth]

I played around with this today, but makes a few metrics functions way more complicated (e.g. lsm_p_core)