Mark hybrid electrons as experimental

• Note that hybrid electrons are considered experimental in input documentation

• Add an ingen warning if using hybrid electrons

Whilst we’ve done a fair amount of testing of the hybrid electron code and have good confidence in this, I think we may want to gather ongoing feedback. At the same time it would be good to get this into the next (imminent) release. As such, here I’m marking the hybrid electron treatment as experimental so that users are aware that they need to take caution if using this feature.