v0.13.0: Delayed Differential Equations

• added support for delayed differential equation (DDE) systems
• a function past(y, tau) is now available for any backend that allows to evaluate a state variable y at time t-tau
• edges with discrete delays that are to be used in combination with an adaptive step-size solver are translated into past calls
• a gallery example was added that demonstrates how to interface the Python package ddeint via a DDE system generated by PyRates
• the Julia backend received support for performing DDE simulations from within PyRates via its interface to DifferentialEquations.jl