Restructured TS map computation

As close to final as it could get today.
This is one of the slowest tests:
```bash
$ time python setup.py test -a "--durations=20"
...
================================================================= slowest 20 test durations ==================================================================
4.32s call     gammapy/spectrum/tests/test_flux_point.py::test_compute_differential_flux_points[table-model]
3.60s call     gammapy/spectrum/tests/test_flux_point.py::test_compute_differential_flux_points[lafferty-model]
3.30s call     gammapy/spectrum/tests/test_flux_point.py::test_compute_differential_flux_points[log_center-model]
1.75s call     gammapy/detect/tests/test_test_statistics.py::test_compute_ts_map
```

Can you make this faster by making the kernel or image smaller?
(I'll fix the other slow tests soon.)