Add Parcels v4 warning at import

[VeckoTheGecko]

I thought that this would also be good to have ahead of the hackathon.

[erikvansebille]

So this warning is raised on every import of Parcels? Is that not overkill? I assume most users run parcels through anaconda, and that the ones who run it through GitHub closing will know that they are on v4. I can imagine it becomes annoying (also for us!) to keep seeing this message

[VeckoTheGecko]

So this warning is raised on every import of Parcels?

If they import Parcels (or a module from Parcels) it will be run once for the session.

I was worried that people who were installing Parcels via conda at the event might forget that they're using the alpha version of Parcels. Though if they do conda env export they will see the version listed there - and I guess since we're promoting installing in a separate environment with a clear name it really isn't that necessary. I don't mind either way. Should I close?

[erikvansebille]

OK I see. Let's then merge and if it becomes too annoying we can always remove it again