Temporal Weighting

[sjordan29]

The transport equation includes $n + \theta$, a temporal weighting factor on the concentrations in the advection and diffusion terms. The current Clearwater-riverine model assumes that $\theta = 1$ (i.e., a first-order fully implicit Backward Euler scheme), but users should be able to specify a $\theta$ of their choice (e.g. $\theta = 0.5$ for a second-order Crank-Nicholson scheme). In general, $X^{n+\theta} = \theta X^{n+1} + (1-\theta)X^n$ (Generalized Euler scheme).

This issue can be closed when:

1. Users can specify $\theta$
2. A comparison has been run to determine how changing $theta$ impacts accuracy for models of various timesteps (e.g., how does a 5 second model's mass balance change when $\theta =1$ versus $\theta = 0.5$?).