Add new constructor to MPoint with (double, double) as parameter

[charlenni]

Add a new constructor to MPoint. This is useful in conjunction with SphericalMercator.FromLatLon() which returns a (double, double) if it is called with double, double as a parameter. So it is possible to create a MPoint directly.

Additionally, add some comments if I have currently hands on.

[pauldendulk]

Nice. I was very happy when the tuple support was improved in C#, because it avoids having to construct another point type. I never used it as much as liked. This is another step in the good direction.