ENH add "specular" mode to th_tth_q solver

[prjemian]

The th_tth_q solver has one mode now in TH Q geometry: https://github.com/prjemian/hklpy2/blob/c48dc578530701f9636b0d7123d8dfb5645a7ecd/hklpy2/backends/th_tth_q.py#L28
This mode only moves tth when q is changed.

A second mode (specular) could move th to -tth for a given q.