Add binary version of topography file.

Also add Python script to produce it.
Modify topography loader to use binary.

This expects a file containing signed 16-bit integers, as generated
using the ascii2bin.py script in DATA/topo_bathy/. Byte swapping should
be handled automatically if necessary.
Tell git to treat topography files as binary.
Merge pull request #167 from QuLogic/binary-topography

Binary topography files