[FEATURE REQUEST] Getter/setter for attribute 'orderPos'

[LdDl]

Is your feature request related to a problem? Please describe.
Currently field

type Vertex struct {
	...
	orderPos int
        ...
}

is unexported.
For extended debugging in external appliactions and import/export function we need to make getter/setter for it.

Describe the solution you'd like and provide pseudocode examples if you can
Pretty simple getter/setter combo:

func (vertex *Vertex) OrderPos() int {
	return vertex.orderPos
}
func (vertex *Vertex) SetOrderPos(orderPos int){
	vertex.orderPos = orderPos
}

Describe alternatives you've considered and provide pseudocode examples if you can
Nope

Additional context
This issue is related to #11

[LdDl]

This is related to #11 since those two unexported fields could be useful for external applications (and export/import graph function)