Motulator

Changes in this release:

- A full-order observer for induction motor added.
- Observers were moved to separate files (im._observers.py and sm._observers.py).
- Some minor bug fixes in the documentation.

This release contains a few minor fixes and improvements:

- Add the path to the saturation date file in an example script 'motulator/examples/obs_vhz/plot_obs_vhz_ctrl_pmsyrm_thor.py'. This addition allows to run the script from other folders.
- Initialization of the PICtrl and ComplexPICtrl classes was fixed: if the reference feedforward gain `k_t` is not provided as a parameter, `k_t = k_p` is used, resulting in the regular 1DOF PI controller. This fix simplifies the usage of these classes as regular PI controllers. See also the added comment in the example script 'motulator/examples/vector/plot_vector_ctrl_im_2kw.py'. For further details regarding the structure of these controllers, see their docstrings as well as the design notes (https://aalto-electric-drives.github.io/motulator/control/speed_ctrl.html).
- Minor improvements in the dosctrings and examples.

This release contains numerous improvements and new features. The most important ones are listed below:

1. The public API was cleaned. The docstrings and the documentation pages were improved. Examples scripts were simplified and reorganized. The naming of instances and classes was also improved by avoiding unnecessary abbreviations as well as avoiding repetition of from the upper hierarchy level.
2. A 2DOF speed controller was refactored and the design notes were added to the documentation. As a special case, the controller can be parametrized as a standard PI controller.
3. A 2DOF PI current controller was refactored and the design notes were added to the documentation. Now, the same base class can be used for both synchronous and asynchronous machines. As a special case, the controller can be parametrized as a standard PI controller.
4. The default machine parameters were removed from the system models and as well as from the control algorithms. Instead, this data is now provided in the example scripts. This change is expected to reduce the risk of user errors.
5. Saturation models were simplified and generalized. The SynchronousMotorSaturated class can be now used with both analytical saturation models and look-up tables. The detailed examples of how to parametrize saturated motor models were moved to the example scripts.
6. To reduce overlapping code, sensored observers were merged with sensorless observers. Design notes were added to the documentation.

This release fixes an issue related to the default load torque functions, making them compatible with array inputs used in post-processing.

This release includes the following improvements:

- Decoupled version of flux-vector control (see, e.g., https://aalto-electric-drives.github.io/motulator/auto_examples/flux_vector/plot_flux_vector_pmsm_2kw.html#sphx-glr-auto-examples-flux-vector-plot-flux-vector-pmsm-2kw-py).
- Reparametrization of the 2DOF PI speed controller, making it more flexible. The description was also added to the documentation (https://aalto-electric-drives.github.io/motulator/2dof_pi_ctrl.html).
- Renaming the number of pole pairs (p -> n_p). This change allows to use p for power in some future methods.
- Improved installation instructions for developers and advanced users (see https://aalto-electric-drives.github.io/motulator/installation.html).

Furthermore, there are various minor improvements.

Improvements in the LUT-based synchronous machine model and in the import method for the SyR-e data