Changes related to E(B-V) guess:

* wrap_bayestar.py: give E(B-V) guess, and return ln(p)
                    for Markov Chain samples.

* data.cpp: If guess is much larger than default E(B-V) guess,
            then resort to default E(B-V) guess. SFD gravely
            overestimates reddening in the plane - could be
            a problem without this fix.