gdaldem color-relief: compute a default color relief if not given

[jidanni]

Feature description

Current gdaldem color-relief usage is

gdaldem color-relief <input_dem> <color_text_file> <output_color_relief_map>

This means the user must spend a long time figuring out how they want the colors, in their <color_text_file>.

Better would just have the gdaldem command dynamically determine a nice color relief, and if the user doesn't like it, yes then they can give a <color_text_file>. I.e, give a now optional [color_text_file].

Yes, a different set of color steps would be produced when gdaldem notices the user has given it a map of the Himalayas, vs. when given a map of Flatsville, Florida. (No one size fits all.)

Also needed would be an option to print on STDOUT the contents of that dynamically produced <color_text_file>, so the user can tweak it if they want, and then use that tweaked version back on the command line.