add a sscan_1d_step() plan

[prjemian]

Update of sscan_1D is not necessary for this issue.

But, it is worthwhile to replace (or create a new sscan_step_1d() plan that accepts a list of:

• detectors
• triggers
• positioners (readback, setpoint, start, finish, npts or step)

and assigns them to sscan record channels, runs the scan, then posts the data to databroker with meaningful names (such as the examples here).

Originally posted by @prjemian in #940 (comment)