Possible improvements to solver base classes

[oparry-ukaea]

• Allow multiple ParticleGroups in PartSysBase?
• Fields are restricted to all have the same number of quad points. Should at least make this check optional, but probably better to remove it altogether
• load_params() copies nektar's unfriendly pattern of relying on the developer to call the parent implementation in the override - find a nicer way to do it
• prefix header guard macros (_NESO*?) to avoid clashes