rgsrmfsmooth

Created a new ftool to correct the XMM RGS response for an extended source. This uses Andy Rasmussen's code included in the xspec v11 model rgsxsrc. Since that model has no free parameters it seems easier just to modify the response matrix before reading it into xspec and that is what this new tool does. This replaces the need to add rgsxsrc to xspec v12.