Add sampling-driven dFBA and make optional deps robust.

[Mohit-Lakra]

    ## Summary
    - add DynamicFBA helper and result container with sampling-based tie-breaking
    - document usage and cite Mahadevan/DFBAlab/COMETS references
    - guard cobra/sparseqr/volestipy/matplotlib imports so the CLI stays usable without optional deps
    - add regression test that exercises biomass/media updates

    ## Testing
    - python -m pytest tests/test_dfba.py