API-interoperability: implement loading of a Grid from a native object

[orbeckst]

Implement creating a Grid from a native object as described in the parent issue #160 for

• mrcfile
• openvdb (once we have add OpenVDB format #141 )

[orbeckst]

We can create sub-issues for MRC and VDB.

[orbeckst]

In principle, we should treat

• filename (str)
• array (array_like) + edges + origin
• grid (Grid)

also as part of how to treat the input argument grid.

To make this approach maintainable and extensible, we should be thinking about breaking the loading process into steps

1. format detection
2. dispatch to a loader that returns either a Grid or a wrapper data structure
3. extract data from loader into the Grid (if necessary)

Ideally, we have the loader functionality bundled with the wrapper classes when possible so that all format-specific code lives in the same place.