Draft: SLB: save pions and neutrals manually, rather than persistreco

Currently for our sprucing lines we're saving particles using persistreco, strictly, we only need the pions. This will later be superseded by !2981 (merged) however for now this reduces our sprucing bandwidth to about 0.79x of what it is currently as a medium-term solution.

We also save the neutrals at this stage, this takes the 0.79x saving and makes it only 0.89%