Changed BoxModel definition #1651

I changed the model definition of the `BoxModels` as proposed in #1651. I decided to include the boundaries in the box i.e. f(x + w/2) = f(x - w/2) = A. Futhermore it was necessary to adapt the kernel module to this changes.
I changed the default discretization of the `BoxKernel` to `linear_interp`, which allows even sized, non shifting kernels. E.g. a kernel with an effective smooting width of 4 pixels has the following array: [0.5, 1, 1, 1, 0.5], as it used to be with the former model definiton.