Add -Ccpt option to sample colors from sample1d and grdxyz

[PaulWessel]

We had no simple way to determine what the colour would be for a given z value (other than under the hood for the usual plotting). This PR lets sample1d and grd2xyz optionally take -Ccpt. This lets sample1d look up the colour based on the input records' last column (or via -i) while grd2xyz will get the colour based on the grid z-values. Both modules then add 4 new columns r, g, b, a to their output.

Note: grd2xyz already used -C[f|i] for some formatting schemes that are rarely used. I changed this to -F and made -C backwards compatible of, course.

gmt math -o1 -T0/30/5 T = | gmt sample1d  -Ct.cpt
0	0	0	127	55
5	0	42	255	55
10	0	212	255	55
15	128	255	128	55
20	255	212	0	55
25	255	42	0	55
30	127	0	0	55

for a random CPT that I made with makecpt -A55 to have a nonzero transparency.

PS. Joaquim and I are working some fixes to the writing of images via GDAL. The goal is to pixel-by-pixel transparency which PostScript cannot do.