Pyrates

- added sign function to the backend that returns the sign of its input (1 or -1)
- improved readthedocs documentation (removed bug with display of math, added new use example for edge templates)
- added a safe guard for defining edge templates: An error is raised now when edge template input variables have the
same name as their source variable.

- minor debugging of the model introduction use examples
- adjustments of the template cheat sheet `template_specification.rst`
- debugged issue in base backend, where file names specified by the users that contained a file ending wre not handled properly
- debugged issue with fortran backend where file names that contained a directory path were not handled properly for module imports
- debugged issue with fortran backend where adjustments of the default auto meta parameters were not applied correctly

- the state variable indices and parameter names returned as the fourth and third return values of `CircuitTemplate.get_run_func`, respectively, now use the frontend variable names instead of the backend variable names
- implemented a method `CircuitIR.get_frontend_varname` that returns the frontend variable name given a backend variable name

- changed the theme of the readthedocs documentation website
- added documentation for all supported backend functions
- added documentation for dependencies and requirements
- added documentation for YAML template structure to the documentation website
- added documentation for mathematical syntax
- added the changelog to the documentation website

- 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`

- 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