Add reverse (I) alias for makecpt

Used 'earth' cmap to test various reversed colormap examples.

pygmt/mathops.py

@@ -7,7 +7,7 @@
 
 
 @fmt_docstring
-@use_alias(C="cmap", T="series", G="truncate", H="output")
+@use_alias(C="cmap", T="series", G="truncate", H="output", I="reverse")
 @kwargs_to_strings(T="sequence", G="sequence")
 def makecpt(**kwargs):
     """
@@ -39,6 +39,13 @@ def makecpt(**kwargs):
         Optional. The file name with extension .cpt to store the generated CPT file.
         If not given or False (default), saves the CPT as the session current CPT.
 
+    reverse (I) : str
+        Set this to True or c [Default] to reverse the sense of color progression in the
+        master CPT. Set this to z to reverse the sign of z-values in the color table.
+        Note that this change of z-direction happens before -G and -T values are used so
+        the latter must be compatible with the changed z-range. See also
+        :gmt-docs:`cookbook/features.html#manipulating-cpts`.
+
     {aliases}
     """
     with Session() as lib:

pygmt/tests/test_makecpt.py

@@ -130,3 +130,36 @@ def test_makecpt_truncated_at_zhigh_only(grid):
     makecpt(cmap="rainbow", truncate=[None, 0.5], series=[-4500, 4500])
     fig.grdimage(grid, projection="W0/6i")
     return fig
+
+
+@pytest.mark.mpl_image_compare
+def test_makecpt_reverse_color_only(grid):
+    """
+    Use static color palette table with its colors reversed
+    """
+    fig = Figure()
+    makecpt(cmap="earth", reverse=True)
+    fig.grdimage(grid, projection="W0/6i")
+    return fig
+
+
+@pytest.mark.mpl_image_compare
+def test_makecpt_reverse_zsign_only(grid):
+    """
+    Use static color palette table with its z-value sign reversed
+    """
+    fig = Figure()
+    makecpt(cmap="earth", reverse="z")
+    fig.grdimage(grid, projection="W0/6i")
+    return fig
+
+
+@pytest.mark.mpl_image_compare
+def test_makecpt_reverse_color_and_zsign(grid):
+    """
+    Use static color palette table with both its colors and z-value sign reversed
+    """
+    fig = Figure()
+    makecpt(cmap="earth", reverse="cz")
+    fig.grdimage(grid, projection="W0/6i")
+    return fig