Use `phets` instead of `phet` everywhere

[benoit74]

See openzim/zim-requests#1389

[kelson42]

We should have "phets" everywhere IMHO, also in the repository name.

[benoit74]

Why ? Domain name is phet.colorado.edu. Logo highlight PhET. What is the logic towards adding a s letter?

[kelson42]

phets = phet simulation. If a choice has to be made between "phet" and "phets", and I would like to achieve coherency, then "phets" seems more appropriate than "phet".

[benoit74]

Let's do this then.

We need to rename:

• the repository
• the Docker image
• the "binary/script" (phets2zim instead of phet2zim)
• the NPMJS package (phets2zim instead of phetscraper)
• the whole codebase / documentation
• ZIM Filename (on the filesystem)

Looks like a tedious change for a small s we could simply remove in ZIM Name (metadata) if you choose the other alternative, but this is your call.