Multi-level GBM trees

Make multi-level GBM trees.  Compute all splits in parallel on a level.
Loop over tree depth; add a tree-depth limiter.
Split out the Histogram into a separate file.
Fill in some unimpl'd for expanding C1 chunks.
Silence javadoc

we should clean up javadocs' errors and then turn back on javadoc
printing... and treat javadoc warnings as real errors.  But meanwhile,
all builds would-be-busted because we got broken javadoc... so silencing
the large pile-o-errors.