Scan modules should check their own dependencies

[hprid]

Currently, we only check if the storage directory exists and issue a warning to run update_dependencies when it doesn't. However, this is not sufficient, e.g., when there is a new scan module or a new dependency for an existing scan module. Therefore each scan module should have a method that checks for their own dependencies, so we can inform the user to run update_dependencies.

This was already discussed in issue #9.