Significant speedup

[dellaert]

Speedups across the board

In DT:

• speed up DT conversion if labels stay

In HGC

• remove double storage: just one tree in HGF base class
• special constructor straight from this base class storage

In HGFG

• prune empty graphs on the fly
• new Result struct to eliminate double compute of negLogK
• use new HGC constructor

Extra

• DT method split : unused for now but could speed up if split could build only two trees instead of three; in that case, we could try building ResultTree as two trees instead, and one of them could be used to create the HGCs directly

[dellaert]

@varunagrawal Hoping to get a quick review so I can cut an alpha version of GTSAM soon.
Before I can do that, I'd also need to ascertain whether memory leak is in GTSAM proper or in your application code. Any information on that?

[varunagrawal]

Couple of comments but LGTM.
Removing the conditionals duplication was on my TODO list (planned to do it after figuring out memory issues) so this is great!