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.

Gauss sampling dtype

See merge request !401

Add one_over() to MultiField

See merge request !400

Switch FFT to scipy.fft

See merge request !398

Fix new finalize method

See merge request !399

Introduce partial insert

See merge request !394

Cosmetics

See merge request !396

Remove standard MPI parallelization

See merge request !387