SIFT tends to crash when loading datasets from the same FCI repeat cycle in separate Open File Wizard calls

[ameraner]

Problem
To reproduce the issue:

• In the Open File Wizard, open the 40 chunks for an FCI repeat cycle
• select one or more datasets to open
• after they opened, reopen the Open File Wizard and select one or more datasets from the same repeat cycle
  After this, and during the Scene opening and loading operations, SIFT has been observed to crash and disappar with a Segfault. However, this does not happen every time consistently.
  This has been observed on a Linux system.

Analysis
The suspect is that it is an issue with multiple accesses to open NetCDF files. Since the Scene gets reinitialised at every Open File Wizard call anyway, possibly a better cleanup/closing/deleting/garbage-collecting of Scene/Filehandlers might fix the issue.

Possibly related to pytroll/satpy#2092