Unit tests for DERIVATIVE prevent change.

[1uc]

Even simple changes like:

• making J row-major
• (X - X_old)/dt = dX -> X = X_old + dt*dX

are practically almost impossible because they require changing the hard coded reference across 1000s of lines of code. For example, https://github.com/BlueBrain/nmodl/blob/6d3b83066cf2c48c045d0953e346b98483aaeab8/test/unit/visitor/sympy_solver.cpp#L1848-L2055

requires changing well over 100 lines of code.