Using calc_direction throughout functions, see #116

For direction_step, direction_to_centroid, direction_to_leader and edge_direction:

• provide alternative sfc interface for direction functions
• if coords are provided, ensure crs is provided (required for lwgeom::st_geod_azimuth)
• if coords are NULL, use default "geometry" sfc interface
• use env argument for programming on data.tables (Fix: Use data.table's env argument #95)

Will require tests added for direction_to_centroid and direction_to_leader after calc_centroid is used throughout (#127)