Handling of missing data files when restoring a steca session

Steca session files store only the paths to data files and not their actual data. Due to this there's a problem when restoring a session file with invalid file paths. By now the only why to rectify this was to edit manually edit the json file per hand. This MR introduced a new feature to handle this problem.

When loading a session file Steca will now check all stored paths to data files before it tries to load them. If it detects an invalid file path it will inform the user and will ask them to provide the valid file path.

While implementing this I made sure the session file with incorrect paths is not changed automatically. The user needs to save the session to a new file to make sure it contains only valid paths.

Resolves #98 (closed)