each reflection should be handled as a Python class

[prjemian]

If all the details of each reflection were handled as a Python class, then many open issues would become much easier to resolve.

Thanks @strempfer for the motivation of this issue!

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• DOC add_reflection(h,k,l) will use current positions #219
• restore_reflections(orientation, fourc) cannot find renamed positioner #215
• cahkl() should report (nicely) that a reflection is not reachable #178
• pa() should identify reflections used to compute UB #176
• Refactor handling of libhkl to be a service #161
• Demo: Write orientation reflections as stream from custom plan. #158
• restore_sample: How to modify existing sample? #157
• Implement/expose simplex method to compute orientation matrix **UB** #154
• Make the __repr__() Hkl.SampleReflection more meaningful #143
• ENH more robust forward calcuation #10
• Issues with hklpy APS-4ID-POLAR/ipython-polar#146
• names of positioners, both pseudos and reals

[prjemian]

Such as #143

[prjemian]

A reflection should be pre-configured with the names of the positioners it uses, both pseudos and reals.

[prjemian]

Notes

A reflection represents a specific coordination between:

• a set of diffractometer angles in real-space (known here as reals), used by a specific diffractometer geometry
• a set of crystalline coordinates in reciprocal-space (known here as pseudos), used by a specific computational engine
• an incident wavelength

The names and ordering of the reals are specified by the diffractometer. The names and ordering of the pseudos are specified by the reciprocal-space computational engine.

A diffractometer may have several samples, each with their own list of reflections. The reciprocal-space computational engine cannot be changed once the diffractometer object is created.

Refactor, considering:

• Each HklSample object has its own set of reflections.
• A HklSample object (created in hkl.calc) needs to know the Diffractometer object to which it belongs.
  • Users call the HklSample.add_reflection() method for new reflections
  • The CalcRecip needs the diffractometer object to which it belongs to create the HklSample

[prjemian]

Note that:

• real axes might be renamed by the user
• additional real axes might be defined which are not used in real:reciprocal space transformations
• additional virtual axes might be defined which are combinations of other real-space axes

[prjemian]

The DCReflection class (in #280) will enable this.