*_ct() family

Common *_ct family of functions including:

• grid_ct: make a grid around each location

• pair_ct: duplicate rows, making pairs of camera traps at each location

• select_ct: select camera trap locations by subsetting (sub) and ranking (rank) locations in each group (by).

pair_ct will likely be used after select_ct but grid_ct may be before or after - though in the case where grid_ct is before select_ct, select_ct will need to flex for means or etc (TODO)

NOTE: grid_ct is previously make_grid (!14 (merged))