Merge branch 'gadget2-part-update' into 'master'

Gadget2 part update

A compilation of changes from @jwillis and myself re-factoring a bit the particle definition in the Gadget version. 
This has knock-on consequences on the other schemes.

Also, a initial vectorization of the Gadget-2 interaction routines will be pushed in with the associated tests. These will be refined and actually used in a separate branch.

See merge request !203