WIP: Interactive handling of Turbomole's define

Cf. #735 (comment 418875942)

Currently requires the proof-of-concept pexpect-based define interface to be (manually) installed as a separate package, but the plan is to copy its code over into ASE once it's cleaned up and fully-featured (would just prefer to have it as a separate package while testing for more focused and faster CI runs).

To-Dos:

• Finish underlying define interface
  • Implement define_str
  • Implement hcore guess
  • Implement use guess
  • Implement restart functionality
  • Clean up / refactor
• Copy proof-of-concept package contents into ASE
• Extend ASE's Turbomole tests a bit
  • Add test that uses initial guess options use and hcore
  • Cover full range of UHF / multiplicity settings
  • Results checking for existing tests?
• Extend docs