Add guard so NuclearModelMap doesn't descend into subalgs

[kjplows]

CAFMaker configuration chokes if this guard isn't enabled.

The culprit is in NuclearModelMap::LoadConfig(), that checks NuclearModel@Pdg=...., as that calls GetConfig() and loops over all entries in GetItemMap(), which expands keys (such as the expected ones e.g. NuclearModel@Pdg=1000180400), treats them like algs, evaluates their config (in this case, SpectralFunc/ApproxElements for the AR25_20i_00_000 tune), and appends keys from that config.

This creates bogus keys like NuclearModel@Pdg=1000180400/AllowElementMatch, which cause UB and segfault.
A tag, v3_06_02_sbn1p1, has been created pointing to commit 7e784bc, and published to SBN CVMFS.
Production workflows using this tag have run successfully on the grid.

Summoning @sjgardiner (who setup most of this original magic 🪄) to review.

Ho Ho Ho! 🎅🌲❄️

[kjplows]

Given this runs end-to-end on production and based on Slack conversations with @sjgardiner I am merging this