PyCBC 'found' events code path is too different

[GarethCabournDavies]

The way that coincs and singles are dealt with in PyCBC live is fairly similar once the events have been found. This should be joined into a single code path so that changes don't creep into one but not the other.

The pycbc live / pycbc offline event file creation would preferably also be put into a single event-file-saving path as well, but that may be a bit difficult.