No support for databases that require username and password so far

[AJKellmann]

Issue:
Some repositories require a username and password for security reasons.
Currently, Graph2VR doesn't support specifying a username and password.

Current Situation:
It is not prepared to specify any password in Graph2VR.

Potential solution:
Allow to specify a username and password.
This could be hardcoded in the settings.txt file (security issues) or requested within the application.

Note: DotNetRDF already has some solutions implemented, see:
https://dotnetrdf.org/docs/2.7.x/user_guide/Configuration-Storage-Providers.html