Replace finite differences with analytical derivatives

[bobmyhill]

The Brosh-Calphad EoS currently uses finite difference to calculate dTheta/dP and the isothermal bulk modulus. For materials with a high bulk modulus, this is inaccurate (and there is no value of deltaP which would be suitable for the range of geologically interesting bulk moduli).

We should

• [ ] replace the finite difference schemes with analytical derivatives in the Brosh-Calphad EoS.
• [ ] replace any other finite difference calculations.