Pyrates

~~~~~~

- 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

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- added support for complex-valued systems
- added model templates for the kuramoto order parameter and the theta
neuron model
- added model templates for the van der pol oscillator and the
stuart-landau oscillator
- added support for Python 3.9
- added new example galleries
- extended pytest library
- added the ``CircuitTemplate.to_yaml`` method that allows to save a
given ``CircuiTemplate`` instance to a YAML definition file
- added the ``CircuitTemplate.add_edges_from_matrix`` method that
allows to connect nodes in a ``CircuiTemplate`` instance via
connectivity matrices
- deleted old, deprecated code fragments
- removed the dependecy on pyparsing

~~~~~~

- updates to changelog and setup.py

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- reworked features:

- Restructured backend

- new backends (torch, Julia)
- sympy-based equation parsing
- improved compute graph
- improved generation of run functions from compute graphs

- Improved frontend

- easier imports
- additional convenience functions for simulations
- less steps from model definition to simulation
- reduced syntax for model definitions

- Removed utility package

- utility packages for parameter optimization, signal analysis
and visualization have been removed from the pyrates main
package
- most utility functionalities have been moved to separate
repositories of the pyrates-neuroscience organization
- less package requirements

- new model templates

- improved structure of the model templates
- New model templates and documentation examples
- new example galleries and jupyter notebooks with hands-on use
examples