Reads and writes a Quadtree.
# S4 method for character
read_quadtree(x)
# S4 method for character,Quadtree
write_quadtree(x, y)character; the filepath to read from or write to
a Quadtree
To read/write a quadtree object, the C++ library cereal is used to
serialize the quadtree and save it to a file. The file extension is
unimportant - it can be anything (I've been using the extension '.qtree').