Create a specification/policy for mapping and broadcasting

[rayegun]

1. When do we map and broadcast in the Matrix way vs the Graph way? How do we expose this to the user succinctly.
2. Backup methods for when cfunc closures occur. We need to:
   1. Determine which functions will accept these binary operators. I would argue mapsparse and map should do this.
   2. Anything else becomes incredibly difficult without reimplementing GraphBLAS which is outside scope for this particular package.
3. Document!!!