Added prune step before compressing global stiffness matrix.

src/threed_beam_fea.cpp

@@ -425,6 +425,7 @@ namespace fea {
         }
  
         // compress global stiffness matrix since all non-zero values have been added.
+        Kg.prune(1.e-14);
         Kg.makeCompressed();
 
         // initialize solver based on whether MKL should be used