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add clarification to einsum docs

See merge request !602

Inverse trigonometric and unitstep nonlinearities

See merge request !598

Do not shadow native `dir`; rename to `direction`

See merge request !596

Add Matern kernel

See merge request !592

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

New pointwise functionality

See merge request !591

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.

Fix doc warnings

See merge request !593