time derivative operator

[martinjrobins]

Summary
This would add an d / dt operator to pybamm to take the time derivative of an expression tree

Motivation
no such operator currently exists. Users can manually add an expression tree corresponding to the time derivative of an expression, but if that expression involves a state variable then there is no way to evaluate an expression that contains the time derivative of the state variable

Additional context
none

[martinjrobins]

I think this would involve:

• a new Symbol class pybamm.VariableDot that represents the derivative of a state variable wrt time, natually the time derivative of a pybamm.Variable results in a pybamm.VariableDot
• a new Symbol class pybamm.StateVariableDot that represents the state time derivative vector , natually the time derivative of a pybamm.StateVector results in a pybamm.StateVectorDot
• discretisation of a pybamm.VariableDot results in a corresponding pybamm.StateVectorDot
• need to change Symbol.evaluate to also take ydot, the derivative of the state vector, as an argument
• raise error in the base_model checks if a pybamm.VariableDot exists in any rhs or algebraic equations (i.e. equations should be in semi-explicit form)