Add alias for blockmean's -S parameter

pygmt/src/blockm.py

@@ -70,6 +70,7 @@ def _blockm(block_method, data, x, y, z, outfile, **kwargs):
 @use_alias(
     I="spacing",
     R="region",
+    S="summary",
     V="verbose",
     a="aspatial",
     b="binary",
@@ -111,6 +112,15 @@ def blockmean(data=None, x=None, y=None, z=None, outfile=None, **kwargs):
 
     {I}
 
+    summary: str
+        [**m**\|\ **n**\|\ **s**\|\ **w**].
+        Type of summary values calculated by blockmean.
+
+        - **n**: report the number of input points inside each block
+        - **s**: report the sum of all z-values inside a block
+        - **w**: report the sum of weights
+        - **m**: report mean value [Default]
+
     {R}
 
     outfile : str