Pyrates

- 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`

- debugged latex equation error in Izhikevich model gallery example
- bugfix in julia backend where a wrong file ending was provided
- added new pytests for the izhikevich model, the python model definition interface and the CircuitIR translation
- updated the readme
- added a new QIF model template that includes conductance-based synapses

- updated parametrization of the Izhikevich model equations
- new introduction gallery example of the Izhikevich model
- updated readme
- updated changelog

- new matlab backend (mainly for code generation, since array conversions between numpy and matlab slow down simulations significantly)
- new mean-field model for the Izhikevich neuron model
- added the natural logarithm `log` as a new backend function that can be used within equations now
- bug fixes:
- removed an issue with the fortran interface to Auto-07p that sometimes resulted in the generation of false parameter vector indices
- removed an issue with synaptic weights of -1 being converted to +1
- removed compatibility issues between new and old `to_yaml` methods

- new documentation galleries/examples introducing the new Kuramoto and theta neuron models
- new theta neuron model with conductance-based synapses
- new tests added to the pytest library to ensure a correct implementation of the Kuramoto and theta model
- changed syntax for absolute value calculation to ensure compatibility with the most recent sympy version (1.10): use "absv(x)" to calculate the absolute value of "x".

- removed bug where vectorized circuits with multiple edges to the same target wre not resolved correctly
- removed bug where creating deepcopies of a `CircuitTemplate` raised an error for scalar-valued models
- added a new gallery example demonstrating different ways of adding delays to models
- added a new gallery example demonstrating the different options to optimize run times of numerical simulations