GridModule: fix leaving processes behind (merge from trunk, https://trac.osgeo.org/grass/changeset/67303)

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@67423 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/python/pygrass/modules/grid/grid.py

@@ -541,6 +541,8 @@ class GridModule(object):
             pool = mltp.Pool(processes=self.processes)
             result = pool.map_async(cmd_exe, self.get_works())
             result.wait()
+            pool.close()
+            pool.join()
             if not result.successful():
                 raise RuntimeError(_("Execution of subprocesses was not successful"))