examples PD_INSTR

[rowlesmr]

from #124

Instrument examples are split between PD_INSTR and PD_INSTR_DETECTOR.

I think the category descriptions need to be updated to point our that the former is source to specimen, and the latter is specimen to detector(s).

[rowlesmr]

From #225

Should PD_INSTR have a link to _diffrn_radiation.id?

That will allow an instrument to know what the incident wavelengths are, and coupled with _pd_instr_detector.instr_id (or successor - see #204 shenanigins), it can also find it's detectors, so it knows everything.

[jamesrhester]

Should PD_INSTR have a link to _diffrn_radiation.id?

Yes. I think a new data name in pd_instr is the right way to go.