Feature extractor changes

This merge request incorporates two things:

1. Removes the /etg_synchronizer folder along with associated executables from the repo. This has been moved a while back to where the feature extractor exists since it makes more sense that it'll live there, and has been changed since then. At the time, where all the code (iDQ+feature extractor) was going to live was still in flux and now that the dust has settled I want to tidy up a bit.
2. Adds an optional keyword argument sample_rate to GstlalHDF5ClassifierData to allow for triggers being produced at higher sample rates, as well as made stride an optional keyword argument now. This by default tries to read hdf5 metadata to get this information and if it's not available (for files produced up until now) will see if these kwargs are available. If none are available, it'll throw an assertion error.
3. Removed reliance on Cache to do processing under the hood in GstlalHDF5ClassifierData, and instead uses a list of CacheEntry instead. Cache was really just a convenience function within glue that did some specialized tools, but was nothing more than a glorified wrapper for a list for what I cared about. I've also switched the backend for CacheEntry from the one in glue.lal to lal.utils which is better moving forward.