Allow upwinding in 1D Heat exchanger

[dallan-keylogic]

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Summary/Motivation:

Breaks off the changes to the HeatExchanger1D from #1382 into their own PR

Changes proposed in this PR:

• Allows upwind difference schemes to be used in the HeatExchanger1D
• Adds warnings about energy conservation when different finite difference schemes are used on either side of the heat exchanger.

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