Add river forcing with adjustable depth range condition for user-set diffusivity in MOM6

[Subrat-Kumar-Mallick]

The modification made here enhance control over diffusivity setting in MOM6. By incorporating river forcing and an adjustable depth range condition in the user_change_diffusivity.F90 file, user can now fine-tune diffusivity parameters based on specific depth ranges and runoff values, improving the model's accuracy in simulating oceanic processes affected by external forcing near the river mouth.