Speedup tests

See merge request !471

Cleanup my ops for NIFTy6 release

See merge request !469

Amend ChangeLog plus minor corrections along the way

See merge request !467

Introduce a numpy.einsum wrapper with derivatives

See merge request !460

Introduce a slicing and a splitting operator

See merge request !461

Increase python version

See merge request !466

Highlight that keys in a multi-domain are sorted

See merge request !465

As the ordering of the random numbers within a multi-field are dependent
on the ordering of the keys, it is highly relevant even for the average
nifty user that keys are sorted. Hence, state so in the docstrings of
MultiDomain, MultiField and in the `from_random` method in sugar.py.

Plus, drop an unused variable.

Among other things, rename `_init2` to the more descriptive name
`_init_wo_preproc` for initialization without (w/o) preprocessing.

Nifty6 einsum conjugate handling

See merge request !463

Nifty6 einsum suggestions pf

See merge request !462