Basic structure of equation tree
Checkpoint, unlocked will soon be procedural
Ready for testing
For the first two levels, generation appears to happen
Merge remote-tracking branch 'origin/story' into story_sm_generates
Organized level 2 nodes

I'm not sure what the best way to do Criteria is.  Also, the updating of
the confidence value is still a black box.
Made first problems easier
Adjustments to some generators,

And removed the extension of EquationGenerator, which is all static
anyway
Merged in student model problem generation