Add function to determine expected outlet temperature.

[spahrenk]

Description

• Provides the following public HPWH member function:
  double getExpectedOutletT_C(const double inletT_C, const double drawVolume_L, const double inletVol2_L = 0., const double inletT2_C = 0.);
• The return value is the outlet temperature (in degC) to be expected at the end of the next time step.
• HPWHsim already performs the draw calculation at the beginning of each time step, while simultaneously updating the tank temperatures. (Processes such as standby loss, inter-node heat transfer, heat addition, and heat-source switching occur subsequently.) This function merely preserves the initial tank temperatures, performs the draw, then restores the tank temperatures to their initial values. The returned value is precisely equal to that produced by the full simulation, using the same input parameters.
• An additional HPWHsim unit test for this feature was added. All cse tests are passed.

Author Progress Checklist:

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