Extend allowed operations in BPX functions

[ejfdickinson]

The use of BPX is currently limited by the restrictive set of mathematical operations that are allowed in FloatFunctionTable inputs.

The schema currently supports the following operations: *, /, -, +, **, exp, tanh.

Here are some suggestions:

• Allow > and < to support piecewise functions as sums, e.g.: (x < 0) * 1 + (x > 0) * 0.5.
• Scope for alternative variable dependencies, such as non-Arrhenius temperature dependence for chosen parameters.
  • A reasonable goal would be that BPX should support the electrolyte parameterisation in Landesfeind-Gasteiger 2019, which is widely used in industry, in which the electrolyte parameters are expressed as general functions $f(c_\mathrm{e}, T)$.
• Support for "MSMR" expressions to write OCP as an explicit inverse function $\theta_k(U_{k,m,\mathrm{ref}})$, which is implemented in PyBaMM as the MSMR model