Expose runoff proxy average as a user-specified parameter in NDR RPI calculation

[claire-simpson]

Description

Allows a user to specify their own average runoff proxy value. More specifically, this PR modifies the calculation of the runoff proxy index (RPI) in the NDR model by exposing the average runoff proxy as an optional user-defined parameter. Previously, RP average was dynamically computed based on the included watershed pixels, leading to inconsistencies when model inputs changed (e.g., different watershed extents or climate scenarios). By allowing users to specify a runoff proxy average, this change ensures consistent RPI values across scenarios, enabling better comparisons between baseline and future conditions.

Changes:

• Introduces new optional model parameter Runoff Proxy Average
• Updates RPI calculation to use the user-defined average runoff proxy (if specified) or the automatically, dynamically calculated average if not specified by the user
• Adds a test and updates documentation

Fixes #1741

Checklist

• Updated HISTORY.rst and link to any relevant issue (if these changes are user-facing)
• Updated the user's guide (if needed)
• Tested the Workbench UI (if relevant)