Create a th-tth solver

[prjemian]

Something missing in the suite of geometries from libhkl is the TwoC engine. Thus, there is no geometry for $\theta-2\theta$ diffractometer geometry.

[prjemian]

• Both forward() and inverse() transformations to $Q$ are simple
• There is one obvious mode: bisector.

[prjemian]

Two different types of hardware:

• all rotary stages (like powder diffraction)
• $\theta$ rotation, $2\theta$ rotation and linear translation (like the APS USAXS)