Fix/post processing

Feature description: Improve the PostProcessing class by cleaning up dependencies to Engine, MetaModel and Model classes. Ensure that the PostProcessing functions can be run multiple times successively

Tasks to complete:

• Remove all PostProcessing.MetaModel and PostProcessing.Model in favor of PostProcessing.Engine.*
• Use Engine properties instead of MetaModel and Model properties where possible (e.g. Model.Output.names)
• Add checks for the PCE-specific functions
• Create a simple GPE that can be used in the tests (Alina)
• Add images and return for the Sobol' indices in addition to the Total Sobol' indices (Rebecca)
• Streamline the output folder generation (folder should be generated in the init if possible)
• Check that all images are generated and saved in the same manner. They should be saved as pdf, but not opened.
• Generalize plot_validation and plot_validation_multi to arbitrary metamodels
• move sobol_calculation to PCE (Rebecca)
• user defined formats for saving the plots (e.g. png)

Checklist

• Tests for the added and changed features
• Add typehints to function signatures
• Check through open todos in changed features
• Add unresolved todos as issues to the git
• Update the Changelog

Possible settings in the tests:

• PCE vs non-PCE (e.g. GPE)
• MetaModel with/without Gaussian properties
• Different dimensionalities of the MetaModel outputs (1/more timesteps, 1/more output keys)