Process MathML used in titles

[drjbarker]

For example 10.1103/physrevb.104.l220402 currently returns

@article{Li_PhysRevB_104_L220402_2021,
  author={Li, Bing and Pajerowski, D. M. and Riberolles, S. X. M. and Ke, Liqin and Yan, J.-Q. and McQueeney, R. J.},
  title={{Quasi}-two-dimensional ferromagnetism and anisotropic interlayer couplings in the magnetic topological insulator <mml:math xmlns:mml="http://www.w3.org/1998/{Math}/{MathML}"><mml:mrow><mml:msub><mml:mi>{MnBi}</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>{Te}</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>},
  journal={Phys. Rev. B},
  volume={104},
  issue={22},
  pages={L220402},
  year={2021},
  month={12},
  publisher={American Physical Society (APS)},
  doi={10.1103/physrevb.104.l220402},
  url={https://dx.doi.org/10.1103/physrevb.104.l220402}
}

So we need to strip the MathML tags out and do something about the subscripts.

[drjbarker]

A good complex example: 10.1103/PhysRev.100.564