Clarify NA meaning in categorical rasters

[jessicaguo]

This supercedes #29. For both bss and strat, there are na's in the averaging, which results in NA in the map. However, we are reserving NA to mean land. Therefore, na's need to be removed in the preprocessing/rasterization.

TODO's:

• [ ] Add special case to make_raster_list() for strat and use "first" instead of "mean"
• [ ] Figure out why bss still has NA's despite using "first"

[keatonwilson]

@jessicaguo I'm going to leave this open and add a task:

• [ ] Incorporate water mask specifically for BSS, given that the new continuous data doesn't contain NAs

[keatonwilson]

Closed by #51