enabled dgamma convolution
enabled dgamma convolution

this also provides a framework easily extensible t other convolution
methods that we do not yet provide

mick-d/nipype,FCP-INDI/nipype,FCP-INDI/nipype,mick-d/nipype,carolFrohlich/nipype,carolFrohlich/nipype,mick-d/nipype,mick-d/nipype,FCP-INDI/nipype,carolFrohlich/nipype,carolFrohlich/nipype,FCP-INDI/nipype