Fix issue in configuration loading

* Fixes an issue where the ConfigurationRegistry would overwrite
  the initial application configuration object.

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Co-authored-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Merge pull request #10 from lnielsen-cern/fix-configuration-handling

Fix issue in configuration loading
Fix handling of missing package resource directory

* Fixes PkgResourcesDirDiscoveryRegistry to properly handle
  missing resource directories.

Reported-by: Javier Martin Montull <javier.martin.montull@cern.ch>
Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Merge pull request #11 from lnielsen-cern/fix-resource-dir-handling

Fix handling of missing package resource directory
Add singleton registry

* Adds SingletonRegistry for ensuring only a single
  object is registered at a given point.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Merge pull request #13 from lnielsen-cern/singleton-registry

Adds singleton registry.