Fixes at the end of ep 4

[josenino95]

Ep 4 in the canonical version is all about raster calculations. In the rAtlas ep 4 I see the calculation made is sea_level <- campus_DEM - 5 to make the raster data and plots make sense.

So all of it works for me up to line 120, but then there's a few things I don't understand and should be fixed in my understanding of the episode:

• Why do we make new bins and another plot in lines 122-135?
• Why do we overlay the original DEM and bathymetry in lines 140-145 and disregard the sea_level* objects we created?
• Line 156 writes the campus_DEM object. But that's the exact same object read at the start of the episode (it had no modifications). Shouldn't we be writing one of the sea_level* objects?