Fix usage of DerivedP.

This is similar to commit 1ae76113ec2872f2595d67f71afd9d79a582f6e5 [formerly e6ffb84de603091ba7b5dbe7fbe1bc7316ed555d]
Otherwise failes to compile on Eigen 3.3


Former-commit-id: d66eb7269ca7877286874b3a05e714c35e11755c

include/igl/shapeup.cpp

@@ -206,7 +206,7 @@ namespace igl
         for (int j=0;j<sudata.SC(i);j++)
           rhs.row(currRow++)=projP.block(i, 3*j, 1,3);
       
-      Eigen::PlainObjectBase<DerivedP> lsrhs=-sudata.At*sudata.W*rhs;
+      DerivedP lsrhs=-sudata.At*sudata.W*rhs;
       MatrixXd Y(0,3), Beq(0,3);  //We do not use the min_quad_solver fixed variables mechanism; they are treated with the closeness energy of ShapeUp.
       min_quad_with_fixed_solve(sudata.solver_data, lsrhs,Y,Beq,currP);