Added active thermal components and usage examples

[eytanadler]

Purpose

This change adds active thermal components to OpenConcept. Additionally, components to help better interface the new components with aircraft models and usage examples have been added. The following components and examples are the notable additions:

• SimpleHeatPump in thermal.py: heat pump with efficiency based on fraction of Carnot efficiency
• HeatPumpWithIntegratedCoolantLoop in thermal.py: SimpleHeatPump integrated into hot side and cold side coolant heat transfer components for easy integration directly into coolant loops
• TwinSeriesHybridElectricPropulsionRefrigerated in thermal_series_hybrid.py: an example of HeatPumpWithIntegratedCoolantLoop implemented into a propulsion model to add active cooling for the battery and electric motor
• HybridTwin_active_thermal.py: an example of the refrigerated propulsion system integrated into a working aircraft model; shows usage of the hot and cold side set temperatures along with the refrigerator bypass switch to control the refrigerator

To implement the above, general purpose utilities were added. Of the ones added, the following are the most generally useful:

• SelectorComp in selector.py: takes in any number of user-specified inputs and routes one to the single output depending on the value of the selector input; used to control the refrigerator bypass
• MaxComp and MinComp in max_min_comp.py: takes a vector input and returns a scalar output that is the max/min of the input

Type of change

What types of change is it? Select the appropriate type(s) that describe this PR

• New feature (non-breaking change which adds functionality)

Testing

Tests have been added for the new components and an integration test was added for the new example aircraft, HybridTwin_active_thermal.

Checklist

Put an x in the boxes that apply.

• [x] I have run unit and regression tests which pass locally with my changes
• [x] I have added new tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation

[coveralls]

Pull Request Test Coverage Report for Build 177

• 301 of 301 (100.0%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.9%) to 87.538%

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Totals
Change from base Build 174:	0.9%
Covered Lines:	3730
Relevant Lines:	4261

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💛 - Coveralls