Implement univariate combinations for distributions.py using * and |

So that in SPML, we can write:
```
GPA ~ 0.3*Uniform(0,1) | 0.7*Bernoulli(p=.8)
```

We could of course preprocessing this expression to:
```
Z ~ bernoulli(p=0.3)
if Z:
    GPA ~ Uniform(0,1)
else:
    GPA ~ Bernoulli(p=.8)
```
but the direct interpretation will be simpler and faster.