Add WGMS dataset to repository

[khsjursen]

Add available cleaned/preprocessed datasets to repository from WGMS.

Citation: WGMS (2026): Fluctuations of Glaciers (FoG) Database. World Glacier Monitoring Service (WGMS), Zurich, Switzerland. https://doi.org/10.5904/wgms-fog-2026-02-10

Cleaned datasets should have the following columns:

YEAR
FROM_DATE
TO_DATE
POINT_LAT
POINT_LON
POINT_BALANCE
POINT_ELEVATION
(POINT_ID)

[albangossard]

This was merged into dev in #177. The license wasn't added so far, so I suggest that we keep this issue open. The license will be added in the SourceManager once it's available.