Typo in docs

See merge request ift/nifty!625

Update density_estimator priors.

See merge request !623

fix numpy deprecation warnings

See merge request !619

Density estimator sugar

See merge request !620

Check that python assert statements are not used

See merge request !621

They are not supposed to be used since they can be deactivated by
`python3 -O ...`

correlated_fields_simple.py: Support fixed zero mode

See merge request !618

This resolves deprecation warnings in numpy>=1.2 .

Unbreak setup when no `git` exectuable in path

See merge request !617

If there is no `git` executable in the path of the installing shell,
the setup will fail because the `setup.py` `write_version` function will
raise a `FileNotFoundError` exception.

Revamp zero-mode handling

See merge request !607

Treat scaling correctly

See merge request !616

Fix VariableCovarianceGaussianEnergy

See merge request !548

This was achieved by exploiting the diagonality of the Fisher metric.
In cases this is not true one needs more samples to increase sensitivity.
Tested that a factor of 2 only in the inverse covariance can now be detected.