detanglement of dependencies through separating out storage backend plugins

[Guy K. Kloss]

DataFinder now has got a large and growing list of dependencies, but not everybody needs to satisfy each and every one of them. Many of them are dependent on the type of storage backend used by DF (e. g. boto for AWS, PySVN for SVN, pythonwebdavlib for WebDAV, ...).

The suggestion is as follows:

 * Split off a core DataFinder application without any data store backends.

 * Make all (meta-) data store backends into plugins, that can be developed and hosted separately, even if they live under the same project.

 * For ease of use, a DataFinder distribution can be created that ships with the/a default plugin (most probably WebDAV).

 * Split off the documentation (e. g. dependencies, build/install/configure notes off to be specific for each plugin.

 * Document for each plugin how it is to be integrated/used with DataFinder.

[Tobias Schlauch (tobias-schlauch)]

Enhanced documentation: https://wiki.sistec.dlr.de/DataFinderOpenSource/RequiredDevPackages

-> Separated core and persistence dependencies
-> Outlined that SVN or WebDAV bindings are at least required for use

[Tobias Schlauch (tobias-schlauch)]

 All stores are already developed as plugins. However, they share some basic utility and interfaces in the persistence package. Although the DataFinder is shipped with every storage back-end, you do not need to satisfy their dependencies if you do not use them. Such a situation is singled with a "Storage interface is not supported" error message.