Use suffix trees to speed up context tree construction

[fabrice-rossi]

Fast VLMC construction can be done using a linear time suffix tree construction algorithm such Ukkonen's algorithm. It make sense to implement it in C++ and to keep the full representation in C++ as well. Steps:

• [x] implement Ukkonen's algorithm
• [x] add counts for support pruning
• [x] integrate the C++ representation for ctx tree
• [x] implement similarity pruning

This should be first limited to VLMC. The cost of COVLMC is dominated by the logistic model estimation and thus supporting a simple transformation of the C++ representation into the current R representation (with nested lists) should be sufficient as a first integration step.

[fabrice-rossi]

This fully implement in branch cpp ("final" commit for this feature cfec292).