Mightier operators

See merge request ift/nifty!414

Gig energy

See merge request ift/nifty!411

Simplifications

See merge request ift/nifty!410

Fix FieldZeroPadder

See merge request !408

getting_started_mf cleanup

See merge request !404

fix NIFTy 5 ASCL citation

See merge request !405

README: add build instructions for docs

See merge request !406

Introduce `nifty6.mpi` namespace

See merge request !395

Scalable energies2

See merge request !402

reverted changes in linearOperator, put changes in scalingoperator instead, fixed bugs and wrote tests:

operator.py: Raise not-implemented in matmul call

See merge request !403

Gordian Edenhofer authored Jan 23, 2020 and Martin Reinecke committed Jan 23, 2020

Contrary to other overloaded operators, the `__matmul__` call apparently
needs to raise an exception instead of returning `NotImplemented`. This
behaviour can be seen for example in:

```python
psp = ift.RGSpace(512)
m = ift.MaskOperator(ift.Field.from_raw(psp, np.random.randint(0, 2, 512)))
m(ift.from_random("normal", m.domain).val) is NotImplemented
```

which should be invalid and raise an exception but previously returned
`True` instead.