[Core] Adding clearing of processes to Clear in AnalysisStage

[loumalouomega]

📝 Description

This PR makes improvements to the analysis_stage.py file.

Mainly it adds the clearing of processes to the Clear method, where right now only the solver is cleared. This is not only an enhancement, but also it is more consistent with the rest of AnalysisStage, where for example the Check method not only checks the solver, but the processes too.

Additionally, it refactors the Check and Clear methods documentation, adding additonal comments and definitions.

@KratosMultiphysics/altair FYI, we already do this in our custom base analysis class, once this is merged we can remove our local implementation.

🆕 Changelog

• Adding clearing of processes to Clear in AnalysisStage

[rubenzorrilla]

Makes sense.