Plus, highlight in detail why the `_Normalization` operator does the
wrong thing when it comes to volumes.

Drop one and rename the other two.

Furthermore, reorder the key-word arguments of the method.

Plus, rename the `halfslope` parameter to `logloghalfslope`.

Adjust the amplitude operator of the Matern kernel for the volume in the
position space as to make the parameters agnostic to the choice of
position space volume.

Fixup get_sqrt()

See merge request !589

Generalize minisanity

See merge request !588

e.g. when the diagonal of a diagonal operator is real and strictly positive.

Fix ducktape behaviour

See merge request !587