Tensor 2 dwi improvements

two improvements:

(1) adding the option to add a baseline image for DWI calculations. Also the b0 is not rescaled to uint anymore (it is not adequate for simulated data). The output DWIs now match perfectly the gradient list, so it is important that the gradient list contain [0,0,0] vectors to have output b0 images.

(2) Tensor2Dwi function: Reading gradients using the itk::GradientReader, and reading the bvalue from a provided usual .bval file instead of a floatingpoint argument