Organization:
- main.cpp runs everything.  It loads up a file (such as test-input),
  and runs 500 random paths, then reports totally phony statistics based on
  how many times each grid square was looked at.

- astar.h contains the prototype A* implementation.  It's terribly slow
  since I use vectors instead of a fib heap for the fringe and either a
  hash table or a flag for the visited list.  Still, it shows how you'd
  write a generic path finding routine: it is a template class that takes
  in a graph (see graph.h) and a heuristic (see heuristic.h)

- heuristics.h shows how to write a heuristic for A*

- graph.h has two types of graph: ones based on a grid, or ones based on
  an adjacency list.  There's also a dummy at the top that shows roughly
  what interface a path finding routine should expect.

- grid.h stores the grid.
- point.h is a 3d int point; an index in the grid, basically

- read.{h,cpp} reads the input and puts it into a grid