Resolve "Add frequency plan"

Closes #24 (closed)

⚠️ Merge after !105 (merged).

• Remove the offsets_freqs parameter
• Add fplan parameter
  • Can use default static locking beatnotes
  • Can read from a frequency-plan file
  • Can use a user-provided dictionary of locking beatnotes
• Change locking condition to use locking beatnotes and not offsets
• Change default 'static' PPRs to match new Keplerian orbit file (v2.0.dev)
• Change default 'static' fplan to produce valid beatnotes for all lock configs
• Add unit tests for frequency plan