Pathsim

Minor fixes, primarily the `Duplex` connection now works properly.

This release adds the capability to solve for the time independent Steady State
- adds special `SteadyState` subclass of `ImplicitSolver`, utilizing the existing optimizers
- bidirectional connnections called `Duplex` that couple two IO pairs as a subclass of `Connection`, useful for RF systems with ports

This release mostly handles a better integration of the AD framework into the solver suite.

- reworked event handling system to act on blocks directly for monitoring and resoution, makes it more versatile
- simulation initialization now utilizes the add_xx methods and is therefore more streamlined
- introduced optim module that houses the AD framework and the solvers for the implicit update equation s for implicit solvers
- the AD framework now has functions for jacobian evaluation and a decorator to compute jacobians automatically
- extended the implicit update equation solver suite by newton-type methods that leverage the AD framework to get the jacobians (experimental and not used currently)
- implicit integrators now support the Value class from the AD framework, this makes stiff systems fully differentiable

- Event handling system with zero-crossing detection and handling of scheduled events with error control, compatible with all integrators
- 25% faster AD by using integer indexing for gradient references in Value class
- blocks, connections and events are the three major system components that are managed by the Simulation class now
- blocks, connections and events have `on` and `off` methods that enable activating and deactivating them (dynamically, based on events or for debugging purposes) in the simulation

- reworked error controllers for the adaptive timestep integrators
- added GEAR type integrators to the solver library (multistep methods with adaptive timestep and adaptive order selection)

- fixed path length estimation for nested subsystems and added tests and an example
- renamed utils.funcs to utils.utils, which makes more sense