implement a faster way to create a LandMask

[amoodie]

Below is an outline for a faster alternative implementation to a LandMask. This method does not use the OAM, and so it is much faster. However, it is not scale-free like the OAM, so it requires an additional scale parameter. Luckily this is easy enough to set, based on the inlet channel width (or some characteristic channel width).

hack_landmask = dm.mask.ElevationMask(
    golf['eta'][-1, :, :],
    elevation_threshold=0)
hack_landmask._mask = morphology.binary_closing(hack_landmask._mask, morphology.disk(5))

5 is the scale parameter here, setting the size of the morphological structural element for the closing.

left is OAP at 1.03 s, right is a hacky mask at 5.5 ms (~200 x faster)

[amoodie]

closed with #63