Added power spectrum tools.

[jejjohnson]

Added some

NOTE: Don't merge until #32, #36 are merged.

---

Main Contrib

• [X] simple isotropic powerspectrum wrapper from xrft
• [X] simple space-time power spectrum wrapper from xrft
• [x] simple isotropic PSD score
• [x] simple space-time PSD score
• [x] example notebooks (with figures)
  • [X] isotropic PSD
  • [X] spacetime PSD
  • [x] isotropic PSD score
  • [x] spacetime PSD score
• [x] Tests...

---

Utils:

• [X] added simple nan interpolation
• [x] utils for 1D threshold (for psd isotropic score)
• [x] utils for 2D threshold (for psd spacetime score)
• [X] made keep_attrs optional for gridding