apply_mask should only take in boolean masks

[ctuguinay]

After talking to @leewujung, we should modify apply_mask to enforce that the masks passed in are boolean masks.

Users can currently pass in masks that are 1/0 and 1/NaN, which stand for True/False; however, more obscure configurations can be passed in, such as arbitrary float/NaN, with the current way that apply_mask handles and applies masks onto Sv/MVBS data, and this makes it a bit confusing for what the user should pass in.