Implement thermodynamic state models

[acodcha]

Implement thermodynamic state models, also known as equations of state:

• PhQ::ThermodynamicStateModel
• PhQ::ThermodynamicStateModel::IdealGas
• PhQ::ThermodynamicStateModel::BoussinesqFluid
• PhQ::ThermodynamicStateModel::StiffenedGas
• PhQ::ThermodynamicStateModel::TaitLiquid
• PhQ::ThermodynamicStateModel::IncompressibleFluid
• PhQ::ThermodynamicStateModel::IncompressibleIsothermalFluid