simple_predictive_sample_unobserved draws randomly even for constrained columns

[riastradh-probcomp]

If you constrain column 3 to be 42, and sample column 3, Crosscat draws randomly instead of returning 42 as one might expect. This results in strange results in bayesdb like:

bayeslite> SIMULATE Murder FROM states_cc GIVEN Murder = 1 LIMIT 4;
Murder
-------------
2.96562662884
9.17781629692
4.15232993703
2.88644682395

[riastradh-probcomp]

There is a candidate fix in the 20150924-riastradh-drawconstraint branch, to force a particular value and not just a particular cluster from which to draw in simple_predictive_sample_unobserved, with an OK from @axch. What remains:

• Automatic test (probably easiest written in bayeslite)
• Determine whether the same issue applies in simple_predictive_sample_observed.

I suspect it doesn't matter for observed rows, because we never ask PREDICT x GIVEN x = 0 expecting it to give 0; in fact bayeslite PREDICT explicitly omits any current value for the requested column in the constraints. But I'm not sure there is no application of simple_predictive_sample_observed for which you might pass a constraint that you're asking about.

[riastradh-probcomp]

20150924-riastradh-drawconstraint now addresses simple_predictive_sample_observed with an essay arguing why this is not sensible and inviting arguments to the contrary before we reject such queries.

[riastradh-probcomp]

Fixed in v0.1.29, abc491c.