Package structure for subtechniques

[jl-wynen]

The current intention is that ESSdiffraction contains code for both powder and single crystal. But at this point, no SXD code has been added. Some existing code is provided in the root ess.diffraction package while some other functions are in ess.diffraction.powder.

It is unclear at this point what code can be shared between the techniques. The current split is based on a guess made in the old ess project and likely needs to be revised.

This is not a huge deal for regular functions and classes as those can easily be imported from wherever they are. But for Sciline providers, the split is more important because domain types used in the common ess.diffraction.providers must match pipelines in both powder and SXD. It is possible that we need to remove the shared provider list and only provide ess.diffraction.{powder,sxd}.providers.

See also this thread: #7 (comment)

[SimonHeybrock]

How about

ess.diffraction  #common
ess.powder
ess.sxd
ess.dream
ess.magic

?