Add ValueCurves from PowerSystems to infrasys

[jerrypotts]

• Added classes for InputOutputCurve, IncrementalCurve, and AverageRateCurve that call function_data classes to build generic cost curves. These classes are analogous to the structs found in PowerSystems
• Added functions to convert between the different value_curves classes (ex: InputOutputToIncremental creates an IncrementalCurve from an existing InputOutputCurve)
• Added functions to function_data to perform calculations for value_curve conversion
• Added additional serialization tests to test_function_data