Set USE_BOOST to default ON

[varunagrawal]

Recently came across this issue where I was trying to wrap a project and I kept getting errors with wrapping the Base noiseModel. Turns out, if USE_BOOST is not set, wrap assumes the use of std::shared_ptr and not boost::shared_ptr.

I propose we set USE_BOOST to be ON by default. This would make sense as long as Boost is a dependency of GTSAM.
The compilation effect should be minimal since boost is only included as a header.

[varunagrawal]

I had to rethink my previous argument. Closing this.