predicate-based finding

[lindahua]

Supports the following syntax

find_to(X > 0, it);  // it is an output iterator
find_to(X > 0, v);  // v is an instance of std::vector<index_t>
find_to(X > 0, I, J);  // I and J are both instances of index_seq

idx = find( x > 0 );  // returns an instance of class dense_col<index_t>

Y = select( A, idx );  // idx is any random access container
Y = select( A,  find(x > 0), colon(2, 3) );

• The find function should accept matrices with bool or mask type.
• The return type should be index_seq
• find function should be a simple wrapper of find_to.