Allow Grids/Scenarios to be created over smaller-than-interconnection areas

# :rocket:

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

### Describe the workflow you want to enable
I wish I could run simulations over geographical regions smaller than an interconnect. For example: running just California without the rest of the Western interconnection, or New York without the reset of the Eastern interconnection.

### Describe your proposed implementation

The simplest possible implementation would be to remove all buses, branches, and generators outside of a given subset of zones that we want to consider. From the modeling perspective, this implicitly assumes that there is no import or export of power along these lines. We could instantiate this using something like: `Grid("Western", zones={"Northern California", "Southern California", ...})`.

In reality, power will flow into and out of a given area, but these imports/exports could be considered 'fixed', and could be modeled using constant or time-varying profiles of injections/withdrawals. For example, something like:
```python
from powersimdata import Scenario
scenario = Scenario()
scenario.set_grid("usa_tamu", "Western")
scenario.set_zones(zones={"Northern California", "Southern California", ...})
scenario.set_import_profiles(df_indexed_by_timestamp_and_columns_are_branch_or_bus_ids)
```

The Scenario would then add pseudo-generators of type `import_export` which have fixed profiles just like hydro generators.

### Additional context
I think this sort of functionality would be useful for state- or ISO/RTO-level analyses, where it's assumed that external conditions are relatively unchanging. This sort of analysis wouldn't capture how changes in powerflow within the study region could impact wheeling of power through other regions, but could allow faster iteration within a region of interest, followed by validation/tuning of the whole interconnection as a final step.