Add Standard Redox Models & Buffers

A set of standard redox models and buffers are added to model.py. These are accessed through the new redox_state and redox_buffer methods in model.Database.

• redox model includes Kress1991 (standard liquid redox model underlying MELTS)
• redox buffers include a large set of standard redox buffers (e.g. NNO, QFM, IW, etc.)
  • multiple examples of each are included where appropriate, and a default parameterization for each buffer is defined
• add alloy phase to accessible database phases
  • this is key for redox-sensitive calculations like metal segregation
  • they can be accessed using MtlS and MtlL for Solid and Liquid Metal alloys, respectively