MueLu: potential aggregation bug

Classic aggregation may not be handling Dirichlet BCs correctly.

[Permalink to lines 80‑100 of `MueLu_AggregationPhase1Algorithm_def.hpp` on the `develop` branch](https://github.com/jhux2/Trilinos/blob/develop/packages/muelu/src/Utils/MueLu_AggregationPhase1Algorithm_def.hpp#L80-L100)