Pyrates

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- 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``

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- 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

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- added gallery example for the izhikevich mean-field model
- updated readme
- updated changelog
- updated default parameterization of the izhikevich model

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- added a matlab backend (mainly for code generation, since simulations
are very slow due to array conversion between numpy and matlab)
- added a mean-field model of the Izhikevich neuron
- small bug fixes

- removed an issue of the fortran interface to Auto-07p that led to
wrong function argument indices being generated
- removed an issue with synaptic weights of -1 being converted to 1
- removed a compatibility issue between old and new versions of the
‘to_yaml’ methods

- added the natural logarithm ‘log’ as backend function