Add support for horizontal and vertical lines from a single value in gmt plot

[Esteban82]

This PR adds support for plotting horizontal and vertical lines from a single coordinate value in gmt plot.

Two new symbols are introduced:

-SY plots a horizontal line spanning the full plot width.
-SX plots a vertical line spanning the full plot height.

Example script:

echo 1 > line.txt
echo 3 >> line.txt
gmt begin linea png
gmt basemap -R0/4/0/4 -Baf -Jx1c
gmt plot line.txt -SX -Wblue gmt plot line.txt -SY -Wred
echo 2 | gmt plot -SY -Wgreen,-
gmt end 

At the moment, the implementation only supports Cartesian coordinates.

The long-term goal is to extend the functionality to geographic and polar coordinate systems, similar to the implementation available in PyGMT to plot parallels and meridians, or arcs and straight lines along radius at a specified azimuth. It may also be possible to add analogous functionality to plot3d in the future, including a -SZ option for drawing reference lines along the Z axis.

Question:

Are the proposed symbol codes -SX and -SY acceptable, or would you prefer different letters?

[seisman]

I'm a little against such a new feature, mainly because preparing a plain text file with two points is easy in GMT CLI (or Bash).

[Esteban82]

I find it a little inconvenient that I need to know the minimum and maximum Y values just to plot a vertical line. For example, in animation 13, we currently use:

printf "%s -2.6\n%s 2.8\n" ${MOVIE_COL0} ${MOVIE_COL0} | gmt plot -W0.5p

This could be simplified to something like:

echo ${MOVIE_COL0} | gmt plot -W0.5p -SY

This way, only the X coordinate needs to be specified, and there is no need to know the Y range or use \n to provide two points.