Allow *Parrays to be created without explicit Standardizer instance

Sometimes it's simpler to ignore the "standardizer" functionality of *Parrays and treat the variable(s) as zero-mean and unit-variance.

Current pattern:
```python
gmb.uparray('y', μ=y_out.mean(axis=1), σ2=y_out.var(axis=1), stdzr=stdzr)
```

Desired pattern:
```python
gmb.uparray('y', μ=y_out.mean(axis=1), σ2=y_out.var(axis=1))  # Internally creates a default Standardizer() instance
```