Distance based shelters refactor

[nihonjinrxs]

Creating a fresh PR here for the refactor work toward distance-based generic locations, with specific implementations of shelters and points of distribution. This replaces PR #7.

Per, the prior PR, here are the tasks to be completed:

• @dstuck Move zip code based input to a separate class & file, keeping the LocationsFinder stuff all Lat/Lon based.
• @nihonjinrxs Refactor to have all base functionality not related to Shelters specifically be applied to Location in a base class, with Shelter specific stuff in a Shelter subclass
• @nihonjinrxs Optimize Location search by calculating a bounding box of Lat/Lon for the mile radius and filtering using that, instead of filtering using the zip codes, then do distance calculation only on those locations within the bounding box
• @dstuck Explore mechanisms for, as a later step, using reverse geocoding in a new input class to find Lat/Lon from user input.
• @nihonjinrxs Implement a subclass for Points of Distribution of food and water.

[dstuck]

As part of the refactor pulling out zipcode, I'm also going to make findShelters accept just a single location input and put the onus on the caller to call it multiple times if they want