Pyrates

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- fixed a bug that caused vectorization to fail if the same operator was used multiple times on a single node
- fixed a bug that caused an error in the generation of unique variable names on nodes with more than 10 operators defined on them
- fixed a bug that caused `CircuitTemplate.clear()` calls to not clear all attributes on a `CircuitTemplate` instace, causing issues with multiple calls of `CircuitTemplate.get_run_func`
- updated the fortran backend to work with the recent changes to the `numpy.f2py` module for generating a modulate that can be imported into python from a fortran file
- fixed a bug in the `ComputeGraph` class of computegraph.py that caused function names to not be updated properly for backend-specific function definitions

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- adjusted the call of the max/min functions: Use `maxi` and `mini` in the equations. Both functions take two input arguments, and return the larger/smaller one, respectively
- updated the PyRates reference in the readme and on the documentation website (using the PLOS CB paper now instead of the arxiv preprint)
- removed a bug where differential equations with a constant right-hand side were not properly handled by the automated compute graph optimization
- resolved an issue with the fortran backend where complex data types were not properly processed during the code generation

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- updated readthedocs configuration file
- added keyword argument `adaptive` to the `CircuitTemplate.get_run_func` method, which allows to indicate whether the generated equation file is expected to be called with an adaptive step-size solver (`adaptive=True`) or not
- reduced computational overhead for the creation and simulation of delayed differential equation systems
- removed a bug where edge attribute dictionaries were changed by mistake during the `CircuitIR` instantiation
- improved working directory management in the backend

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- simplified automated generation of unique variable names (recursive calls etc. were replaced with look-up tables)
- improved variable passing between different operators within a node. Less additional variables are now created, thus reducing the memory load

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- fixed bug in fortran backend where the NPAR parameter for Auto-07p files was not properly set
- improved code readability in fortran backend
- moved selection of output variables from the results of a numerical simulation from the backend to the computegraph, thus reducing the amount of variables that had to be passed between the different classes
- after each simulation, the value of all state variables in the compute graph is updated to the value at the final simulation step
- added functionalities to the `CircuitTemplate` that allow to remember the state of all network variables from a previous simulation, even if a new backend is chosen for function generation or more simulations

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- added a background input parameter to the izhikevich population template
- updated the documentation example for parameter sweeps to account for recent changes in the keyword arguments to the `grid_search` function
- changed keyword argument `vectorization` of the function `grid_search` to `vectorize`, to be consistent with the naming of the same argument in `CircuitTemplate.run`
- updated the `CircuitTemplate.add_edges_from_matrix` method to allow for edges that connect separate network nodes