Update armadillo to version 10. The main changes since the last release are:

• C++11 is now the minimum required C++ standard;
• faster handling of compound expressions by trimatu() and trimatl();
• faster sparse matrix addition, subtraction and element-wise multiplication;
• expanded sparse submatrix views to handle the non-contiguous form of X.cols(vector_of_column_indices);
• expanded eigs_sym() and eigs_gen() with optional fine-grained parameters.