how to specify mongodb instance, error starting FGLab

Hi and thanks for FGLab,
I was wondering how exactly to start FGLab?
I've followed the steps of installation but when I try to start the lab I get the following errors:
```
> FGLab@0.7.6 start /home/user/FGLab
> node lab.js

Error: No MongoDB instance specified
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! FGLab@0.7.6 start: `node lab.js`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the FGLab@0.7.6 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! /home/user/.npm/_logs/2019-02-13T19_29_46_962Z-debug.log

```
Also how do you specify if your `db` is in a different directory than the working one?

Start with lab.js and the npm start script, then trace how startup obtains the MongoDB instance. Determine how an instance or a database directory outside the working directory is specified; done means the supported startup path is clear and the reported error is resolved or documented.

Written by the indexing model from the issue text.