isdisjoint is python2.6

Add docstrings

Remove dependence function by default. Can now sort by anything.
Does not respect dependence by default.

This version is simpler and more declarative. May be slower

Fails tests because tests have order hard-coded

Np17

NEWS:

documented, speed up(50% and renamed _dfs_toposort (now _contains_cycles)
Rename FunctionGraph.nodes -> apply_nodes. The old nodes still work, but get warned if used.
Made a parent class called Node for Apply and Variable class that are both FunctionGraph nodes

Use the async gpu kernel call by default.

make is clear that the allocated space is not needed.