boolean flatten: bug fix in dfs search

(this is not intended to (and doesn't) address the issue with
NodeBuilder limit)
DecisionWeightAttr
decisionThreshold option
--decision-use-weight, --decision-random-weight=N
added simple node weight computation for bv.
decision: fix design to do partial explorations

* make findSplitterRec and all related helper functions' return
  type trivalued, to be able to distinguish between
  "partial exploration" vs "done exploration but found nothing"

* keep additional data structure to remember to what extent the
  partial exploration has been completed so not to repeat it. we
  can use this to make multiple passes on formula with arbritrary
  order of thresholds for exploration
decision: option to auto compute weight of boolean structure
decision::atom
trail size stat
stat bv::weightComputationTimer
fix potential bug with threshold
split theory and decision requests
sat --favor-internal=(factor as double)
DECISION_WEIGHT_INTERNAL_USR1
--dump=tree:{weight,weightfull} creates dot graph with weights

* tree:weight only prints the boolean structure of the formula
* tree:weightfull also goes into the term computations (if one exists)
bugfixes in usr1 auto weight computation