Rename helper function to avoid name collision with class method

[bestill3]

Branch name: main

File affected: /influpaint/utils/season_axis.py

Description:
There are two methods/functions with the same name in this module. One is class-level method get_season_week(self, ts) which is an instance method and belongs to the class namespace. The other is get_season_week(ts, start_month, start_day) which is a standalone function (module-level), it lives in the global namespace.

This is valid but slightly risky design because:

• same name could cause cognitive confusion for developers
• debugging stack traces become harder to read

Cleaner alternative is to rename the standalone function as below:
def compute_season_week(ts, start_month, start_day)

Then the class method becomes

def get_season_week(self, ts):
    return compute_season_week(ts, ...)

I will let Joseph review and decide if we want to make this change.