Improve unit handling

[spahrenk]

Description

• Changes unit-handling throughout HPWHsim. Independent physical constants relevant to HPWHsim are reduced to a short list of exact or reliable values, from which others are then derived. These are defined in file units.hpp, which provides inline initialization of conversion maps and defines partially specialized types for containing values and vectors with unit information.
• The conversion mechanism is established in 'unity.hpp'. A static std::map is retained for each type of units that provides conversion to/from a reference unit. No association between units of different types (e.g., 'm/s' = 'm' / 's') was deemed necessary to improve units organization within HPWHsim, while maintaining performance.
• Values with units are specializations of ScaleVal and ScaleOffsetVal, and are retained as doubles. Effort was made to enable easy accessibility of the underlying double, while retaining explicit construction. The internal value (double), or a reference to this value, can be accessed with the () operator.
• The same guidelines were applied to vectors. These are retained internally as std::vector<double> (e.g., rather than std::vector<ScaleVal<...>>).
• Every model test reference was replaced, due to generally negligible changes. Differences in yearly test results are somewhat larger, but appear to be within normal tolerances (~ 0.1%). An additional unit test for units conversions is added.