check-installed does not consider multi-users systems

The current `check-installed` implementation does only verify if a planner has been installed by the current user:
https://github.com/AI-Planning/planutils/blob/ef05ad69276b687b22a29768c6628640d92d28ab/planutils/package_installation.py#L31-L32

As the planners are installed globally in `/usr/local/lib/python3.7/dist-packages/planutils-0.2.6-py3.7.egg/planutils/packages`, a planner will always been installed for every user, but the corresponding name is only inserted in the settings.json of the user, who issued the install command.

I do suggest either move the settings.json to the global directory (at least the `installed` part) or add individual "check-installed" scripts to the planner folders in `/usr/local/lib/python3.7/dist-packages/planutils-0.2.6-py3.7.egg/planutils/packages`,
e.g. [...]/planutils/packages/enhsp-2020/check-install
```
#!/bin/bash
test -f "$(dirname $0)/enhsp-2020.sif"
exit $? #Return return code of test
```

PS: As installing planners requires root permissions, but executing is file with user permissions, I had to modify my setting.json manually at the moment.