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

Renaming of variables in cell and space struct

Renamed 'h' in cell struct to 'width' and renamed 'h' and 'ih' in the space struct to 'width' and 'iwidth'.



See merge request !206

Isothermal potential

added some more external potentials to gravity, plus a test for energy and angular momentum conservation in the IsothermalPotential/GravityOnly example.

See merge request !205

Renamed 'h' in cell struct to 'width' and renamed 'h' and 'ih' in the space struct to 'width' and 'iwidth'.