Balanced POD

[FHoltorf]

This could be an extension to #2

Both are quite simple mathematically.

Original balanced POD method: https://arc.aiaa.org/doi/abs/10.2514/2.1570

Slight improvement of the original method: https://www.sciencedirect.com/science/article/pii/S0021999112002045

[FHoltorf]

Very concise and approachable description (and improves Wilcox and Peraire's work from an algorithmic standpoint!) https://www.worldscientific.com/doi/abs/10.1142/S0218127405012429