Add first test for slicing

This MR will allow to slice OfflineTracks, so they return OfflineTracks again (for consistent behaviour). Currently an OfflineTrack is returned, regardless of the slice type (tracks[23] vs tracks[5:23]).

CHANGELOG.rst

@@ -4,6 +4,10 @@ Unreleased changes
 
 Version 0
 ---------
+0.6.2 / 2019-12-22
+~~~~~~~~~~~~~~~~~~
+* Fixes slicing of ``OfflineTracks``
+
 0.6.1 / 2019-12-21
 ~~~~~~~~~~~~~~~~~~
 * Minor cleanup