Partially completes obsidianmd#16

This changes the handling of location strings by checking whether they are plus codes before processing them as lat/lon pairs. It also adds `open-location-code` as a dependency.
## Overview of Plus Codes

Plus Codes are a way to represent geographical areas by using a string of characters and offer a system for addressing that is more human friendly than raw latitude and longitude coordinates.

For example, the plus code **9C3XGVJC+2W** describes the location of [the British Library](https://plus.codes/9C3XGVJC+2W) in London. Compare this to the latitude and longitude values 51.530002, -0.127709, here written as Decimal Degrees, one of the many ways in which latitude and longitude are used.

| Notation                      | Example (British Library)                                                                     | Comment                                       |
| ----------------------------- | --------------------------------------------------------------------------------------------- | --------------------------------------------- |
| Decimal Degrees (DD)          | 51.530002, -0.127709<br>*OR*<br>51.530002°, -0.127709°<br>*OR*<br>51.530002 N, 0.127709 W<br> | GIS                                           |
| Degrees Minutes Seconds (DMS) | 51°31′48.01″N, 00°07′39.75″W<br>*OR*<br>51°31′48″N, 00°07′40″W                                | Navigation and cartography                    |
| Degrees Decimal Minutes (DDM) | 51° 31.8' N, 12° 46.254' W                                                                    | Electronic navigation (aviation and maritime) |
| ISO 6709                      | +51.530002-000.127709/                                                                        | Can include altitude                          |
| Well-Known Text (WKT)         | POINT(-0.127709 51.530002)                                                                    | Written with longitude first!                 |

A plus code **does** encode location coordinates, but in a way that is unambiguous, culturally independent, and easy to decode. It also represents an **area** rather than a single point. This allows **flexible precision**: the first four digits describe a 1° lat by 1° lon area, and each subsequent digit reduces the area further and further. A 10 digit code describes an area approximately 14m x 14m, and adding 5 more digits describes an area of **4 x 14mm**.
#### See also
- [Open Location Code specification](https://github.com/google/open-location-code/blob/main/Documentation/Specification/olc_definition.adoc)
- [plus.codes](https://plus.codes/) demo

Replace separate generate_plus_code_files function with a unified
generate_test_files that accepts a pluscode_count argument (default 20),
outputting both coordinate and Plus Code files to the same directory.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>