Add a feature to apply spatial reduction to an input Grid

# :rocket:

- [ ] Is your feature request essential for your project?

### Describe the workflow you want to enable
I wish I could use a predefined spatial reduction to create a reduced-order Grid from an higher-order Grid. This is the spatial equivalent of #55, which applies a temporal reduction.

### Describe your proposed implementation
A function would take an input Grid and a data structure of spatial mapping to create an output Grid. For example, we may want to reduce our nodal system to a zonal system, so in that case we would need to define which buses map to which zones, and potentially how to determine the aggregate transfer capacity between zones (if this cannot be accurately inferred by the bus-to-zone mapping).

The output does not necessarily need to contain _all_ of the information that's in a Grid object, since it will be used to generate Switch inputs, not run a production cost model (e.g., we don't need to worry about branch impedances).

This pre-defined spatial mapping would possibly be used by an output stage to 'un-reduce' the network.

### Additional context
This is probably a `v0.3` or beyond task, but I wanted to get an issue started to collect discussion on high-level design (e.g. data structures for inputs & outputs).