Enable construction of reference state from user-defined profiles

Description

The reference state is currently constructed from a finite set of hardcoded temperature profiles, see TemperatureProfiles.jl. These profiles have been chosen because the pressure field can be constructed analytically imposing hydrostatic balance.

In the more general case, a reference state can be constructed from a user-defined profile of a given thermodynamic variable (e.g., pressure, density or temperature). Of particular interest to users would be the possibility to construct a reference state given a (virtual) potential temperature profile, θ=θ(z).

The code does not currently support easily this option because it requires performing the following integral to recover the pressure field p,

(pᵏ - pᵏ₀)/k = -gpᵏ₀ ∫ 1/(Rθ) dz, k = Rd/Cp

Ideally, we would want to construct a PrescribedTemperatureProfile function that returns the pointwise p given a function θ=θ(z). For that, we need to be able to integrate in TemperatureProfiles.jl.