Add cross_product example to documentation (#665)

* Cross_Product Example

* Update cross_product

- Removes the first empty line
- Uses doxygen commands
- Adds result at the end as comments
- Mentions that the second point is undefined in Example 1

* Update Cross_Product

- Added a missing `

* Update imports.qbk

- Adds [import src/examples/arithmetic/cross_product.cpp]

* Create cross_product.qbk

* Update cross_product.hpp

Refers cross_product example

* Delete cross_product.qbk

Uneccessary

* Update

Deletes .qbk reference

* Update make_qbk.py

* Rename cross_product to cross_product.cpp

* Update

-Shifts example reference to line 115-116

Co-authored-by: Vissarion Fisikopoulos <fisikop@gmail.com>

doc/imports.qbk

@@ -85,6 +85,7 @@
 [import src/examples/algorithms/unique.cpp]
 [import src/examples/algorithms/within.cpp]
 
+[import src/examples/arithmetic/cross_product.cpp]
 [import src/examples/arithmetic/dot_product.cpp]
 
 [import src/examples/core/coordinate_type.cpp]

doc/make_qbk.py

@@ -106,6 +106,8 @@ algorithms = ["append", "assign", "make", "clear"
     , "relation", "reverse","simplify", "sym_difference", "touches"
     , "transform", "union", "unique", "within"]
 
+arithmetic = ["cross_product"]
+
 access_functions = ["get", "set", "exterior_ring", "interior_rings"
     , "num_points", "num_interior_rings", "num_geometries"]
     

doc/src/examples/arithmetic/cross_product.cpp

@@ -0,0 +1,57 @@
+// Boost.Geometry
+// QuickBook Example
+
+// Copyright (c) 2020, Aditya Mohan
+
+// Use, modification and distribution is subject to the Boost Software License,
+// Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+//[cross_product
+//` Calculate the cross product of two points
+
+
+#include <iostream>
+#include <boost/geometry.hpp>
+#include <boost/geometry/arithmetic/cross_product.hpp>
+
+namespace bg = boost::geometry; /*< Convenient namespace alias >*/
+
+int main()
+{
+     //Example 1 2D Vector
+     
+     bg::model::point<double, 2, bg::cs::cartesian> p1(7.0, 2.0);
+     bg::model::point<double, 2, bg::cs::cartesian> p2(4.0, 5.0);
+           
+     bg::model::point<double, 2, bg::cs::cartesian> r1;
+       
+     r1 = bg::cross_product(p1,p2);
+
+     std::cout << "Cross Product 1: "<< r1.get<0>() << std::endl;  //Note that the second point (r1.get<1>) would be undefined in this case
+        
+     //Example 2 - 3D Vector 
+     
+     bg::model::point<double, 3, bg::cs::cartesian> p3(4.0, 6.0, 5.0);
+     bg::model::point<double, 3, bg::cs::cartesian> p4(7.0, 2.0, 3.0);
+           
+     bg::model::point<double, 3, bg::cs::cartesian> r2;
+       
+     r2 = bg::cross_product(p3,p4);
+
+     std::cout << "Cross Product 2: ("<< r2.get<0>() <<","<< r2.get<1>() <<","<< r2.get<2>() << ")"<< std::endl; 
+     
+     return 0;
+}
+
+//]
+
+//[cross_product_output
+/*`
+Output:
+[pre
+Cross Product 1: 27
+Cross Product 2: (8,23,-34)
+]
+*/
+//]

include/boost/geometry/arithmetic/cross_product.hpp

@@ -90,7 +90,8 @@ struct cross_product<3>
 \param p1 first vector
 \param p2 second vector
 \return the cross product vector
- */
+
+*/
 template <typename ResultP, typename P1, typename P2>
 inline ResultP cross_product(P1 const& p1, P2 const& p2)
 {
@@ -110,6 +111,9 @@ inline ResultP cross_product(P1 const& p1, P2 const& p2)
 \param p1 first vector
 \param p2 second vector
 \return the cross product vector
+
+\qbk{[heading Examples]}
+\qbk{[cross_product] [cross_product_output]}
 */
 template <typename P>
 inline P cross_product(P const& p1, P const& p2)