Add orbital selection and orbital lightcurve capabilities

This changes add the possibility of performing a selection on orbital phase in order to compute orbitally folded likelihood lightcurves. There are a few new configuration options and two new scripts: ``enrico_orblc`` and ``enrico_plot_orblc``. The ``OrbitalLC`` class is subclassed from the Lightcurve class with only a few changes in the preparation and plotting. Most of the additions are in the ``Observation`` class, where a quite complex gtmktime filtering is carried out based on the ``phasemin`` and ``phasemax`` configuration options (and a hack is needed to allow for more than 20 orbits selected, see the docstring on ``_phase_MkTime``).