Pyrates

- added generic method for state variable indexing to circuit.py that is used for all edge-related indexing operations now (replacing multiple, slightly different implementations at various places in circuit.py)
- added an alternative compute graph class that can be used to generate function files that do not perform in-place manipulations of the vectorfield `dy` but instead just create a new variable. This is relevant for gradient-based optimization.
- improved the modularity of the `ComputeGraph`
- added a method `add_import` to the backend that allows adding import statements to the top of a function file
- added a backend function `concatenate` that can be used in equation strings now in order to combine vectorized variables
- removed a bug where calling `clear_frontend_caches` did not clear all IR caches properly

- added support for models with vectorized state-variables
- improved performance of edge operations
- more detailed output about returned function arguments when calling `CircuitTemplate.get_run_func`
- improved memory consumption during model initialization
- complex-valued models use complex variable types for all variables and parameters now, to prevent type conversions
- added a new method `CircuitTemplate.get_var` that allows users to access backend variables after calling `CircuitTemplate.get_run_func`
- added automated reduction of vectorized constants, if all constants are identical
- added possibility to pass iterables to `CircuitTemplate.update_var`, thus allowing to update vectorized variables in one go
- updated `CircuitTemplate.add_edges_from_matrix` such that only edges with non-zero weights are added to the `CircuitTemplate` instance

- run-function generating method `ComputeGraph.to_func()` now returns the keys of the function arguments together with the arguments (tuple of size 3 is the new return value)
- implemented a method `CircuitTemplate.get_variable_positions` that allows to get the indices of state variables within the system state vector

Updated changelog to account for changes between v0.14.0 and v0.14.1

- added more Izhikevich mean-field model versions (biophysical and unitless, distributed background currents and spike thresholds)
- moved all Izhikevich model templates into a separate yaml file
- improved documentation gallery examples (removed typos from equations, debugged image embeddings)

- added Heun's method as a new method for solving the DE systems
- Heun's method has been integrated with all backends
- correct implementation of Heun's method is ensured by testing it against Eulers method and a Runge-Kutta method
- added Heun's method to the simulations gallery example
- improved the backend implementation of choosing between different solvers (less overlap between different backends)
- improved documentation of the numerical solvers used in the simulations gallery example