fix `assign_lake_flag.py` to work with different directory structures

[iamdonovan]

This PR fixes the following issues in assign_lake_flag.py:

• checks whether the RGI regional shapefiles are stored in a single directory, or in individual subdirectories. If neither is the case, a FileNotFoundError is raised with a message.
• clarifies that "RGI v7 region name" is the full filename of the shapefile, minus the extension (e.g., RGI2000-v7.0-G-01_alaska).