Provide grb::select( [out] Matrix, [in] Matrix, grb::operator(r,c,v) -> bool )

[byjtew]

The select primitive(s) is required for multiple incoming algorithms.
For instance to threshold under/over a specified value, or in tril in order to filter depending on the coordinates.

The idea would be to generalise the incoming tril, triu and diag primitives.

• grb::tril( L, A ) -> grb::select( L, A, grb::boolean_operators::is_lower_part<RIT, CIT>() )
• grb::triu( U, A ) -> grb::select( U, A, grb::boolean_operators::is_upper_part<RIT, CIT>() )
• grb::diag( D, A ) -> grb::select( D, A, grb::boolean_operators::is_diag<RIT, CIT>() )
• grb::diag( A ) -> grb::select( A, grb::boolean_operators::is_diag<RIT, CIT>() )

The diag boolean_operator would then have the following apply function:

template< 
  typename RIT, typename CIT, typename T,
  typename BoolOrEquivalent = bool
>
BoolOrEquivalent apply( RIT i, CIT j, T value ) {
  return static_cast<BoolOrEquivalent>( i == j );
}

The naming is to be discussed, of course.