Fix a minor bug in full_trace_over_estimator

There is a special case when both the trace and the estimator are exactly
zero. In that case the ratio 0/0 is ill-defined numerically but should really
be 1 because the estimator provides a correct value.
Add version.py to get git hash
A naive attempt to improve statistics by introducing a new probability
distribution.
Histogram to analyse statistics of G(\tau).
asymm_bath test: plot also Delta(\tau)
Experiments on reweighting.
Merge branch 'igor' of github.com:TRIQS/cthyb into igor