Independently allow cubic spline interpolation option in PerformancePrecisionTradeoffs

[shorowit]

Issue overview

E+ 9.6 offers a new cubic spline interpolation option for PerformancePrecisionTradeoffs. In trying to use it, I discovered that it can only be accessed by selecting Mode06 or higher. It'd be nice if this input could be independently offered so that you aren't required to use everything else included in Mode06. As far as I'm aware, every other option in this object can be independently selected so that a user has more flexibility than the preset modes.

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[mjwitte]

At the same time, it would be nice to use a consistent format for the warnings from PerformancePrecisiontTradeoffs. Here's an example of some of the current warnings:

    ** Warning ** Due to PerformancePrecisionTradeoffs Override Mode, internal variable MaxZoneTempDiff will be set to 1.0 .
    ** Warning ** Due to PerformancePrecisionTradeoffs Override Mode, internal variable MaxAllowedDelTemp will be set to 0.1 .
    ** Warning ** PerformancePrecisionTradeoffs: Coil Direct Solution simulation is selected.
    ** Warning ** Due to the use of the fast simulation mode, the time step for simulation and averaging window of sizing is overwritten to one hour. Original user inputs for averaging window and timestep are no longer used.