Rank selection issue in _compute_svd

[loiseaujc]

Hi guys,

I have noted a small issue with the rank selection in _compute_svd(X, svd_rank) when user prescribes a desired cumulative energy (i.e. a truncation error). Line 242 of dmdbase.py, the code reads

cumulative_energy = np.cumsum(s / s.sum())

However, the energy in POD analysis is related to the square of the singular values, not the singular values themselves. The code should instead read

cumulative_energy = np.cumsum( s**2 / (s**2).sum() )

This is pretty straightforward to modify so should I send a PR or do you guys wanna handle it?

[mtezzele]

You are right, thank you! I will fix this, no problem!

[mtezzele]

Fixed in #89