Unify FileTable and DatabaseAccess

[pmrv]

Started #336, the general recipe for this to go through the generic interface

https://github.com/pyiron/pyiron_base/blob/master/pyiron_base/database/jobtable.py

and then move each function there to an abstract method in IsDatabase and split the implementation to methods in DatabaseAccess and FileTable. Since the latter is newer, this mostly ends up moving stuff to methods in the former.

[samwaseda]

Create one PR per method of jobtable.py, transfer it to filetable.py.

As a preferred alternative to overwriting these methods, SQL alchemy in memory -> Initialization and tests missing