Adopted contributions by Christian.

Adopted contributions by Christian.
* Contributions by Christian, resulting in vast speed and memory improvements
  for large factor graphs:
  - Sparse implementation of nodes->edge conversion table _E12ind in bipgraph.h
  - New FactorGraph constructor that constructs from given ranges of factors
    and variables
  - Optimization of FactorGraph constructors
* FactorGraph constructors no longer check for short loops and for
  negative entries. Also, the normtype is now Prob::NORMPROB by default.
* Moved everything into namespace "dai"