[Therion] sd for a vertical pole

[Tarquin Wilton-Jones]

> In Therion, this generates an error "invalid quantity -- plumb".
> 
> sd length 0.001 metres
> sd plumb 0.125 degrees
> gps surveystart 2.000 - DOWN

I wrote a bunch of tests. Therion is missing the required "sd plumb"
functionality, and has no equivalent. I have filed this as:
https://github.com/therion/therion/issues/445

The only workaround is to use:

sd length 0.001 metres
sd clino 0.125 degrees
gps surveystart 2.000 - -90

but this will then generate a warning from Cavern saying that the
compass bearing may not be omitted. (It will still work, however.)