Adding operator overloads

There is a large number of operator overloads, to facilitate strongly typed computations such as Speed speed = Length.FromMeters(100) / TimeSpan.FromSeconds(9);.

1. Put operator overload in Length.extra.cs if the first parameter is Length
2. Add a short xmldoc summary as per the example below. You can add more descriptions if it is useful.

The reason is to have a consistent place to find the operator overloads and the compiler complains if the containing type/file does not match any of the parameters of the operator overload method.

/UnitsNet/CustomCode/Quantities/Length.extra.cs

/// <summary>Get <see cref="Speed"/> from <see cref="Length"/> divided by <see cref="TimeSpan"/>.</summary>
public static Speed operator /(Length length, TimeSpan timeSpan)
{
    return Speed.FromMetersPerSecond(length.Meters / timeSpan.TotalSeconds);
}