doc: add minimal structure to SkeletonDataset class

[neutrinoceros]

PR Summary

• Init required instance attributes in order to leave a valid Dataset structure.
• minor doc update (hopefully making the instructions a little clearer)

edit:
the changes to SkeletonDataset make it so the following code pass without error

yt.load("myfile.from.a.new.format")

where the only changes required from the author of a new frontend are

• adding the frontend to yt/frontends/api.py
• implementing NewDataset._is_valid()

[matthewturk]

Nice!! Looking forward to review, but this looks good so far.

[neutrinoceros]

Actually I was expecting to do more with this as I went along implementing my new frontend (PR still not open), but I didn't need to. This is now ready for review.

[munkm]

One minor comment but everything looks good to me.

[munkm]

It might be worth suggesting what a good value would be for this in a code that doesn't have time dependence. Since it is required, a helpful suggestion might be good here.

[neutrinoceros]

Good point. This didn't occur to me. Actually I don't know why it's required at all. I should look for the reason first.
Do you think ˋ0.0` would make sense here ?

[matthewturk]

This seems fine to me. My suggestion would be a time of zero for default, but setting to -1 makes it so that they explicitly have to set it if they don't want some weird value.

[neutrinoceros]

So I'm down with whatever default value you deem most sensible @munkm , though I don't know what would be a helpful suggestion here :/

[munkm]

I think leaving it as -1 in the code but having an inline comment that we suggest 0 would work.

[neutrinoceros]

done !