Add References to Implemented Publications

[VisDan93]

In my opinion, the references to the implemented publications, such as:

• Uncertainty-Aware Principal Component Analysis,
• Uncertainty-Aware Seasonal-Trend Decomposition Based on Loess,
• Uncertainty-Aware Multidimensional Scaling,
• Uncertainty-Aware Spectral Visualization,
• Uncertainty-Aware PCA for Arbitrarily Distributed Data Modeled by Gaussian Mixture Models,
• ...,

need to be added at least to the corresponding example Jupyter notebooks but possibly as well to the functions, e.g., uadapy/dr/uapca.py.

For a complete list of methods and their references, I would propose adding a table to the README file showing "| Method | Paper Ref |"

[hageldave]

@marinaevers