Pyrates

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- added ``__getitem__`` methods on all frontend template classes that
allow for a less convoluted examination of the major properties of
the template classes
- added pytests that test these new features
- users can now quickly access each node on ``CircuitTemplate``, each
operator on ``NodeTemplate`` and ``EdgeTemplate``, and each variable
on ``OperatorTemplate``

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- added class for interactive grid search results visualization to
utility
- changed organization of the pandas DataFrames that ``grid-search``
returns: Each different parameterization of the model appears only
once in the ``param_grid.index`` and the ``results`` DataFrame uses a
full hierarchical column organization.
- The pandas DataFrame returned by ``CircuitTemplate.run`` uses a fully
hierarchical column organization now: Every node hierarchy level is a
separate level in the column index hierarchy.
- minor docstring improvements
- fixed bug in edge equation setup where a wrong index was provided to
the target variable sometimes
- fixed bug in variable updating that occurred for ``numpy.ndarray``
variables where the ``shape`` attribute was an empty tuple
- applied all changed to the gallery examples in the documentation

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- 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

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