Add national land cover dataset color table (#518)

* added nlcd color table

* added nlcd to color descriptions

lib/gis/colors.desc

@@ -28,6 +28,7 @@ kelvin: blue to red for temperature in Kelvin scale
 magma: perceptually uniform sequential color table magma
 ndvi: Normalized Difference Vegetation Index colors
 ndwi: Normalized Difference Water Index colors
+nlcd: US National Land Cover Dataset colors
 oranges: white to orange
 plasma: perceptually uniform sequential color table plasma
 population: color table covering human population classification breaks

lib/gis/colors/nlcd

@@ -0,0 +1,22 @@
+# color table for USGS National Land Cover Dataset
+0 255 255 255
+11 72:109:162
+12 231:239:252
+21 225:205:206
+22 220:152:129
+23 241:1:0
+24 171:1:1
+31 179:175:164
+41 108:169:102
+42 29:101:51
+43 189:204:147
+51 175:150:60
+52 209:187:130
+71 237:236:205
+72 208:209:129
+73 164:204:81
+74 130:186:157
+81 221:216:62
+82 174:114:41
+90 187:215:237
+95 113:164:193