Create natural_vector

Create natural_vector as a subtype of integer_vector

• Syntax close to constraining the integer_vector. Maybe one of the following is necessary
• subtype natural_vector is integer_vector (<>) (range 0 to integer'right) ; -- seems to be consistent with partial constraints
  • subtype natural_vector is integer_vector (<>) of (range 0 to integer'right) ;
  • subtype natural_vector is integer_vector ((range 0 to integer'right)) ;