Add recommended citation banner to looper

[egull]

Summary

• add a recommended-citation box to the looper copyright output
• cite the ALPS paper, the looper implementation paper, and the three algorithm papers listed in the ALPS documentation

Testing

• cmake --build build --target loop -j2

[egull]

Assigning @Pollet as our resident QMC guy.

[Ooolab]

Independent verification

I built the PR and ran the resulting loop binary on a real simulation input, and checked each citation. Everything works as claimed.

Build & runtime test

• Rebuilt the loop target with this change (incremental CMake build, matching the PR's stated test) — compiles cleanly; the macro edit (string-literal concatenation with line continuations) is valid and follows the file's pre-existing style.
• Ran the fresh binary on a real input (a coupled ladders task from the mc-08 tutorial). The banner renders exactly as intended at the top of the run, between the looper copyright and the parapack block, and also appears via loop --license. Since this block prints at the start of every simulation run, the citation reaches exactly the audience it should.
• The box is perfectly aligned — all 10 lines are exactly 68 characters.
• The simulation itself completes normally (exit 0, all tasks evaluated); output files are untouched by the banner, so downstream pyalps parsing is unaffected (it reads the XML/HDF5 result files, not stdout).

Citations checked

All five references are bibliographically correct:

• [1] J. Stat. Mech. (2011) P05001 — the ALPS 2.0 paper; pleasingly, the ALPS library's own banner printed ~10 lines below in the same output cites the identical paper.
• [2] Todo & Kato, PRL 87, 047203 (2001) — the looper implementation paper; also listed in applications/qmc/looper/doc/index.html.
• [3] Evertz, Lana & Marcu, PRL 70, 875 (1993) — the original loop algorithm.
• [4] Beard & Wiese, PRL 77, 5130 (1996) — continuous-time loops.
• [5] Evertz, Adv. Phys. 52, 1 (2003) — the loop-algorithm review; also in the looper doc.

Two cosmetic nitpicks (take or leave)

1. [1] JSTAT (2011) P05001; ends with a stray semicolon while the other entries have no terminator.
2. "JSTAT" differs in style from "J. Stat. Mech." used by the ALPS library banner printed directly below it in the same output — using the same abbreviation would read more consistently.

Neither affects function — the PR does what it says.

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