Added method to return the true HSI matrix
The HSI matrix being returned by the BGR2HSI::convert() method was of
type Mat_<Vec3b>, but the H, S and I values were of type double. This
led to a lot of information being lost in the conversion from double to
uchar. The BGR2HSI::returnTrueHSI() method acts as a get() method for
the Mat_<Vec3d> true_HSI private data member. The true_HSI matrix stores
the actual double values that were obtained after the conversion.