Added additional input arguments

A small PR to add arguments to phantomas_dwi script.

I found it useful to play with lambda 1,2,3 and to specify the output file name.

Maybe it would be better to use a JSON file to describe all parameter of the model (gm/wm/csf t1-t2-rho, gm/csf diffusivity, lambdas, te-tr...) What do you think?
Just a question: Why are you adding a metavar parameter?