long wavelength approximation

This patch:

• adds in a few reference PSDs and detector orientations.
• changes the way we automatically cache known PSDs and Detectors
• adds __repr__ functionality to PSDs, Detectors, and Network
• implements a TwoArmDetector response that does not follow the long-wavelength approximation.
  • this implementation was tested by reproducing figures from Essick et al, PRD 96, 084004 (2017).