v0.15.0: Support for vectorized state variables

• 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