clean up & fix more miscellaneous things (#177)

* remove all assets

* no need to import Random & DataStructures in GridWorlds module

* fix type signature of act! & get_pretty_tile_map

* use sample_empty_position instead of sample_two_positions_without_replacement in SingleRoomUndirected

* update sample_empty_postion methods

* rearrange abstract_grid_world.jl

* fix method signature of get_pretty_sub_tile_map

Author: Sid-Bhatia-0