Try moving ST_Graph to h2spatialext :(

This won't work because the functions in h2network depend on ST_Graph!
Fix ST_Graph displacement

Actually, it will work! The downside is that in the h2network unit
tests, I have to simulate ST_Graph's behavior rather than calling it
directly.

The advantage is that Cobertura correctly analyzes ST_Graph's code
coverage, and ST_Graph seems to belong in h2spatial-ext anyway.