Fix #1080

Sources of uncertainty labels and tooltips based on following documents:

Riese, C.; Moser, D.; Belluardo, G.; Ingenhoven, P. (2018) Uncertainties in PV System Yield Predictions and Assessments, PVPS Report IEA-PVPS T13-12:2018, International Energy Agency (IEA) Photovoltaic Power Systems Programme (PVPS). https://iea-pvps.org/wp-content/uploads/2020/01/Uncertainties_in_PV_System_Yield_Predictions_and_Assessments_by_Task_13.pdf

Hansen, C. (2021) Conceptual Approach. Internal document.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• This change modifies variables in existing compute modules. Please see Checking for PySAM Incompatible API Changes.

Checklist:

If you have added a new compute module in a SSC pull request related to this one, be sure to check the Process Requirements.

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings