Align prepmd Packaging and Release Workflow with Other CCPBioSim Repositories

[harryswift01]

Summary

This PR updates prepmd’s packaging and release setup to align more closely with the approach used by other CCPBioSim repos.

Changes

Package configuration

• Switched the project build backend to Flit.
• Added dynamic versioning using prepmd.__version__.
• Simplified prepmd/__init__.py by removing eager imports of all submodules.
• Retained the existing prepmd, runmd, and prep-license command-line entry points.

Conda packaging

• Updated the Conda recipe to build the package using Flit.
• Added the required prepmd runtime dependencies.
• Kept openff-toolkit as a Conda runtime dependency.
• Added import and command-line checks to the Conda package tests.
• Configured dependency resolution through conda-forge.

Release workflow

• Added automated version updates for prepmd/__init__.py and CITATION.cff.
• Added automatic Git tagging and GitHub release creation.
• Added automated Conda package building and publishing to Anaconda.org.
• Kept the Conda recipe version as a placeholder that is replaced during the release workflow.

Impact

• Makes prepmd’s packaging and release process more consistent with other CCPBioSim repos.
• Avoids importing every prepmd module during a basic package import.
• Reduces import failures caused by optional or heavyweight dependencies.
• Provides a more consistent and repeatable Conda release process.