Derenzo phantom

main+function to create a Derenzo phantom for Gate v6.1 PET/SPECT simulations

Derenzo.m : extracts centroids and radii of the circles that make up the Derenzo phantom (needs derenzo.png to work)
derenzo_main.m : example of application; gives as output a .txt file with the Gate commands needed to define the geometry and the sources of the phantom. Also produces a plot as a preview of the final phantom.
Main parameters (activity, size, material of phantom..) can be changed according to needs.
Suggestions and bug reports are welcome!