Make QubitSearchEngine an abstract class
Make QubitSearchEngine an abstract class

I'm still unclear on the purpose of QubitSearchEngine. I'd like to
define a number of requirements and some basic funcitonality like
logging for search engine implementors (like arElasticSearchPlugin)
but I don't know if the extra work is worth it. Anyways, an abstract
class seems to fit better than an interface in this case.

Surfrdan/atom,artefactual/binder,kyfr59/atom-cg35,Surfrdan/atom,CENDARI/atom,artefactual/atom,artefactual/atom,artefactual/binder,CENDARI/atom,kyfr59/atom-cg35,artefactual/binder,Surfrdan/atom,artefactual/atom,CENDARI/atom,kyfr59/atom-cg35,artefactual/binder