Refactor river biochemical processes

[barneydobson]

Currently difficult to understand and hard to test. Needs to be split into sensible functions.

[barneydobson]

calculate_discharge, which calls biochemical_processes, is currently commented out in the distribute function

[liuly12]

In the River node, these parameters were in CatchWat for solids simulation but not used in the current version. There will be very little chance to reuse them again so removed by commit c0a192120002ad6e5ae65cf90718f641b0167e0a in #81

        self.qbank_365_days = [1e6, 1e6]  # [m3/day] store outflow in the previous year
        self.qbank = (
            1e6  # [m3/day] bankfull flow = second largest outflow in the previous year
        )
        self.qbankcorrpar = 0.001  # [-] correction coefficient for qbank flow
        self.sedexppar = 1  # [-]
        self.EPC0 = 0.05 * constants.MG_L_TO_KG_M3  # [mg/l]
        self.kd_s = 0 * constants.MG_L_TO_KG_M3  # 6 * 1e-6, # [kg/m3]
        self.kadsdes_s = 2  # 0.9, # [-]
        self.Dsed = 0.2  # [m]