Make bottom_mask() return boolean (True = above bottom) to match apply_mask() #250
Description
bottom_mask() currently returns 0/1 integers with 1 = Bottom and below, 0 = above. Since apply_mask() in echopype now expects boolean masks (True/False), this is inconsistent. Let’s switch bottom_mask() to return booleans with True = above the bottom (keep/valid) and False = at/below the bottom (mask/invalid).