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.
3. Add a unit test case and place it in the equivalent file LengthTests.cs.

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.

Example

UnitsNet/CustomCode/Quantities/Length.extra.cs

/// <summary>Get <see cref="Volume"/> from <see cref="Length"/> times <see cref="Area"/>.</summary>
public static Volume operator *(Length length, Area area)
{
    return Volume.FromCubicMeters(area.SquareMeters * length.Meters);
}

UnitsNet.Tests/CustomCode/LengthTests.cs

[Fact]
public void LengthTimesAreaEqualsVolume()
{
    Volume volume = Length.FromMeters(3) * Area.FromSquareMeters(9);
    Assert.Equal(volume, Volume.FromCubicMeters(27));
}