added exception class MatricesCantBeAddedOrSubstracted
added throw of exception MatricesCantBeAddedOrSubstracted to method
operator+(with Matrix)
removed test MatrixAddWithAnotherMatrix and all it's cases
fixed case SizesAreNotEqual of test MatrixOperatorPlus because
of adding exception throw to method operator+(with Matrix)
fixed case SizesAreNotEqual at test MatrixOperatorMinus because
of adding throw of exception MatricesCantBeAddedOrSubstracted
to method operator-(with Matrix)
added requirements section