LzoStream.Flush shouldn't throw NotSupportedException

[DRKV333]

Currently calling LzoStream.Flush() trows a NotSupportedException:

lzo.net/src/LzoStream.cs

Lines 476 to 479 in 6c84b61

	public override void Flush()
	{
	throw new NotSupportedException();
	}

This can cause problems, because certain other .NET library function will always attempt to call Flush after they are done with the stream. Specifically, I ran into this issue while trying to use a CryptoStream to hash the contents of an LzoStream.

According to the documentation, Stream.Flush() should normally be implemented as an empty method for read-only streams. https://docs.microsoft.com/en-us/dotnet/api/system.io.stream.flush