Release v0.3.4.0

This release yet again updates the version of poseidon-hs (to v1.2.1.0). This enables xerxes to read Poseidon packages of version 2.7.1. We also switched to a newer stack resolver version and updated the GitHub Actions for automatic testing.

One new feature was added: The code for the admixpops subcommand was refactored to include a new input flag --inChunks. This allows to sample not just on the level of individual SNPs for the construction of artificial ancestry-chimeras, but also in chunks, so longer stretches of SNPs of length --chunkSize. Please note that admixpops remains experimental with no guarantees of correctness.