More consistent quantity names

[seanpearsonuk]

Goal: reduce the divergence between the formatting of quantity names across the APIs.

Expressions format for physical variables is camel case: "AbsolutePressure". That also shows up in reduction code, e.g.,

>>> reduction.area_integrated_average(
  expr="AbsolutePressure",
  locations=locations
)
0.07665669042888468

data model APIs format is separate capitalized words, e.g., "Velocity", "Mass Flow". These show up in the unitQuantity attribute.

The unit-quantity attribute in the settings API uses lower-case names, e.g., "velocity". For multiple-word names, this presumably extends thus: "specific-heat" (in the Fluent format).

Note that both unitQuantity and unit-quantity will be exposed via methods with the same name across various APIs.

Attributes for actual concrete physical variables in the settings API have names like velocity, thermal_conductivity and specific_heat. For the data model APIs the names are like OperatingPressure.