Currently, the same sympy symbols are used for parsing of the state updaters in ExplicitStateUpdater and the equations. This has the unintended effect that the identifiers f and g in equations is interpreted as functions and not as a variables.
More generally, identifiers in the namespace should be parsed according to their role (e.g. if f refers to a Function specifier, it should be interpreted as a function, if it refers to a variable, it should be interpreted as a symbol).
Be more precise about symbols in the state updater description and
symbols in the equation itself -- should fix problems with using
variable names such as x, f, or g. Closes #24.