Wrap meca (#516)

* Wrap meca command
* Write tests for various meca input formats
* Add example to gallery

Author: tjnewton

doc/api/index.rst

@@ -35,6 +35,7 @@ Plotting data and laying out the map:
     Figure.image
     Figure.shift_origin
     Figure.text
+    Figure.meca
 
 Color palette table generation:
 

examples/gallery/plot/meca.py

@@ -0,0 +1,33 @@
+"""
+Focal mechanisms
+----------------
+
+The :meth:`pygmt.Figure.meca` method can plot focal mechanisms, or beachballs.
+We can specify the focal mechanism nodal planes or moment tensor components as
+a dict using the ``spec`` argument (or they can be specified as a 1d or 2d array,
+or within a specified file). The size of plotted beachballs can be specified
+using the ``scale`` argument.
+"""
+
+import pygmt
+
+fig = pygmt.Figure()
+
+# generate a basemap near Washington state showing coastlines, land, and water
+fig.coast(
+    region=[-125, -122, 47, 49],
+    projection="M6c",
+    land="grey",
+    water="lightblue",
+    shorelines=True,
+    resolution="f",
+    frame="a",
+)
+
+# store focal mechanisms parameters in a dict
+focal_mechanism = dict(strike=330, dip=30, rake=90, magnitude=3)
+
+# pass the focal mechanism data to meca in addition to the scale and event location
+fig.meca(focal_mechanism, scale="1c", longitude=-124.3, latitude=48.1, depth=12.0)
+
+fig.show()