PALS Version Attribute

[ax3l]

In openPMD, we have an important attribute that self-describes the version of the standard used in each file.
I would propose we do the same for PALS, increasing reproducibility and clarifying what to expect when opening a PALS file.

I propose to use semantic versioning for the standard.

[ax3l]

place for the attribute defined in #152.

Now we just need to agree on a versioning scheme once we get closer to a first PALS release candidate.