ctools

I realised that in the current version of ctbin, we get a segmentation fault when running on an observation container with different energy thresholds.
The problem occurs in line ctbin.cpp:.526, where we query for the usage of the energy bin, depending on the event energy.
The variable iebin is not checked before checking usage[iebin]. However, in case the cube is only a subset of the complete energy range, an event energy may be outside the cube energy range. Consequently the line before

int iebin = m_ebounds.index(event->energy());

can result in iebin=-1. Therefore, we need to add a check and replace the if statement by:

if (iebin == -1 || !usage[iebin]) {
    num_outside_ebds++;
    continue;
}

Note: we need to make sure that the first part of the condition is evaluated first. I recall we had some issues with the order how c++ executes the || statements.