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?
metavars are only use to generate the help. I found the help more clear if written --export_fod BASIS "..."  than --export_fod EXPORT_FOD "...", since the basis must be specified. For --order_sh ORDER was just shorter than --order_sh ORDER_SH. I don't mind so much, what do you think?