Split meca out from base_plotting.py

Author: weiji14

pygmt/figure/__init__.py

@@ -67,6 +67,9 @@ def __init__(self):
         self._preview_dir = TemporaryDirectory(prefix=self._name + "-preview-")
         self._activate_figure()
 
+    # GMT Supplementary modules
+    from .meca import meca
+
     def __del__(self):
         # Clean up the temporary directory that stores the previews
         if hasattr(self, "_preview_dir"):