Modify Distance and Direction Equations

[threeal]

Currently the distance and the direction equations in the Point classess are as follows:

static double distance_between(const Point & point_a, const Point & point_b);
static double angle_between(const Point & point_a, const Point & point_b);

double magnitude() const;
double direction() const;

To make the usage simpler, instead of calling:

auto distance = keisan::Point2::distance_between(a, b);

It's better to move the distance_between and angle_between function to the namespace scope, not in the class scope as a static function. So the function could be simply just be called like this:

auto distance = keisan::distance_between(a, b);

Because how c++ works, it's possible to have multiple function name with different argument type and result type. Hence it's fine to put it on global scope.

Also, sometimes it's also more convenient to just call a distance and a direction equation like this:

auto distance = a.distance_to(b);

That's why it's better to also add function to calculate distance and direction equation between self and other Point. So aside from previously mentioned functions, also add the following function to the Point classes:

double distance_to(const Point & point);
double direction_to(const Point & point);