BUG: fix and test ModelData.set_effect_prior

This makes the `set_effect_prior` method of the ModelData class function;  it was never completed or tested previously.  Thanks to Ver for motivating me to do it.

Now code such as
```
 	dm.set_effect_prior('p', 'x_cigc', dict(dist='TruncatedNormal', mu=0., sigma=1., lower=-10., upper=0.))
```
should work and set an informative prior on the `x_cigc` effect coefficient, constraining it to be at most zero.