ParrayPlotter called on matplotlib function can't handle specifying axes

Current pattern:

```python
plt.sca(ax)
pp = gmb.ParrayPlotter(X, Y, z)
pp(plt.contourf, levels=20, cmap='pink', norm=norm)
pp.colorbar(ax=ax)
```

Desired pattern:

```python
pp = gmb.ParrayPlotter(X, Y, z)
pp(plt.contourf, levels=20, cmap='pink', norm=norm, ax=ax)
pp.colorbar(ax=ax)
```