Random matrices

[lindahua]

Introduce a set of random matrix expressions

template<class Engine, class Distribution>  class random_expr;

Distr distr;
rand_mat( m, n,  distr, eng ); // ==> random_expr<Eng, Distr>
rand_mat( shape, distr, eng );

randu<T> ( m, n, eng );  // ==> random_expr<Eng, uniform_real_distribution<T> > 
randu<T> ( shape, eng ); 

randn<T> ( m, n, deg ); // ==> random_expr<Eng, normal_distribution<T> >
randn<T> ( shape, eng );

Notes:

• The value type should be inferrable from Distribution at compile-time.
• random_expr should implement IEWiseMatrix