Add legend symbol for confidence lines

[PaulWessel]

Description of proposed changes

Both plot and plot3d can plot lines with build-in confidence bands. This is done via the option -L+b|d|D that reads extra column(s) to build the envelope around the line (drawn with -Wpen) and the envelope is painted via -Gfill. The envelope may also take an outline pen via -L's +ppen modifier. However, we had no way to handle this in a legend. The sad and painful process was to draw them as two separate things, then struggle a bit with a manual legend setup to draw a line on top of the rectangle for the envelope fill.

This PR detects the -L situation and basically issues to auto calls for legend entries, the first using the special symbol L for a rectangle that should not see a legend position change after it, and then the line as the second symbol which then overprints the rectangle. With this PR, simple examples like this:

#!/bin/bash
# Test CL legend
gmt math -T0/3/0.1 -N3/0 T 2 POW -C2 10 DIV 0.2 ADD = line.txt
gmt begin CL
	gmt plot -R-0.5/3.5/-1/11 -JX15c -B -L+d -W1p -Gpink line.txt -l"My trend and 2@~s@~ bounds"+jTL
gmt end show

will give a legend entry like this:

[seisman]

Looks great, but why is the rectangle a little shorter than the line?

[PaulWessel]

Couple of reasons:

• I happen to like it over the alternative
• This required no code changes for the rectangle, it is the same used for polygons

To make it the same width as the line would require more coding of a special case. Seems not worth it to me.

[seisman]

Couple of reasons:

• I happen to like it over the alternative
• This required no code changes for the rectangle, it is the same used for polygons

To make it the same width as the line would require more coding of a special case. Seems not worth it to me.

Sounds reasonable to me.