WIP Explore Latex in subplot tags

[PaulWessel]

See this forum post for background. Not straightforward since we run a special PSL function to set those. Tried the obvious (relying on the other places we handle this) but get a crashing PostScript. Will follow up.

[PaulWessel]

For testing, here is a small subplot that fails due to Latex code:

#!/bin/bash
gmt begin Figure png
  gmt set FONT_TAG 16p,Times-Italic
  gmt subplot begin 1x1 -Fs6i -A+o-0.2c/0 -JW180/6i -Rg
    gmt subplot set 0,0 -A"@[\nabla^2@["
    gmt basemap ${eq_proj} -B0
  gmt subplot end
gmt end show

I will update the subplot documentation with a note that Latex expressions in the -A is currently not allowed.

[PaulWessel]

See #8016 for documentation warning.