Alias truncate (G) for colorbar

Included one test for checking the truncation works to a min/max range. Also made sure region (R) is parsed as a slash separated sequence string.

Author: weiji14

pygmt/base_plotting.py

@@ -126,8 +126,16 @@ def coast(self, **kwargs):
             lib.call_module("coast", build_arg_string(kwargs))
 
     @fmt_docstring
-    @use_alias(R="region", J="projection", B="frame", C="cmap", D="position", F="box")
-    @kwargs_to_strings()
+    @use_alias(
+        R="region",
+        J="projection",
+        B="frame",
+        C="cmap",
+        D="position",
+        F="box",
+        G="truncate",
+    )
+    @kwargs_to_strings(R="sequence", G="sequence")
     def colorbar(self, **kwargs):
         """
         Plot a gray or color scale-bar on maps.

pygmt/tests/baseline/test_colorbar_truncated_to_zlow_zhigh.png

@@ -128,3 +128,13 @@ def test_colorbar_box_with_offset_background():
     fig = Figure()
     fig.colorbar(cmap="rainbow", box="+s5p/-5p", position="x0c/0c+w1c/0.5c")
     return fig
+
+
+@pytest.mark.mpl_image_compare
+def test_colorbar_truncated_to_zlow_zhigh():
+    """
+    Create colorbar truncated to z-low and z-high.
+    """
+    fig = Figure()
+    fig.colorbar(cmap="rainbow", truncate=[0.15, 0.85], position="x0c/0c+w2c/0.5c")
+    return fig