Bug fixes across OQ-VMTK modules to improve performance

[mouayed-nafeh]

This pull request integrates a series of fixes, improvements, and suggestions to the OQ-VMTK modules and source code, together with updates to the associated demonstration notebooks and documentation.

The OQ-VMTK developers would like to acknowledge the valuable suggestions and feedback provided by two anonymous reviewers during the peer-review process for Earthquake Spectra.

The main changes included in this pull request are:

• Replace “spectral” with “pseudo-spectral” when referring to acceleration and velocity in imcalculator.py, and update the references associated with the different intensity measure types supported.
• Generalise the slfgenerator.py module by removing predefined regression types and relying exclusively on empirical distributions generated through Monte Carlo simulations. Users can now define and fit a regression form during the post-processing stage.
• Remove the semi-empirical coefficient-of-variation calculation based on Silva (2019) from postprocessor.py as a method for loss uncertainty quantification. Loss uncertainty is now propagated explicitly from the underlying fragility functions and probabilistic consequence models.
• Remove the fragility-function rotation functionality from postprocessor.py.
• Combine the calculation of average annual damage probability and average annual loss into a single function signature, eliminating redundancy and reducing code size.
• Simplify the SDOF-to-MDOF calibration procedure and structural-dynamics implementation in calibration.py.
• Add support for optional user-defined Pinching4 parameters for nonlinear zero-length elements, while retaining a default set of parameters.
• Update the collapse detection signal for SPO, CPO, and NRHA analyses to enable early termination once the capacity of any storey is exceeded.
• Add unit tests
• Update demonstration notebooks and documentation page