Change in calculating derv2.

[antoniaelek]

Hello Arthur,
I've started using this great project of yours earlier today and I've noticed it throws System.IndexOutOfRangeException in OSPCResult.Create method when there are only two files to be compared.
Exception ocurres on line
result.POI_Similarity = lst[lst.CalcDerv2().MaxIndex()];
because CalcDerv2() returns an empty array and MaxIndex() then returns the default -1.
Also, CalcDerv2() would throw System.OverflowException when passed an array containing less than two elements.
I thought that a straightforward workaround to this would be simply not to try to calculate POI_Similarity when unable to calulate the derivation, so I've made some changes to avoid having these error messages show up. It would be great if If you could review them.
Best regards,
Antonia