Link to TNGLab

[cbyrohl]

We need to include a link and instructions to run on TNGLab.

[dnelson86]

Under "Large Datasets" you say "If you want to run the code below, you need access to the full TNG simulation dataset."

Here you can just suggest that the easiest way is to use the TNG-Lab.

[dnelson86]

On all the other such pages, you say "If you want to run the code below, consider using the demo data as described here."

You can also suggest the alternate option of using the TNG-Lab, such that you don't have to download anything.

After these updates, can close this issue.