Page edited + Info on Bain groups added

[AzdiarGazder]

Hi Ralf,

As requested by you, I've made some additions to the page: https://mtex-toolbox.github.io/MartensiteVariants.html

I've added in some explanations for the variant, crystallographic packet and Bain group Ids. I've also made some minor edits to the page.

Hope you find everything satisfactory.

Warm regards, Azdi