Return fields if compute_mask=False, and dependences

[yyang35]

changes

• Return predicted fields if compute_mask=False rather than empty list.
  • This flag should provide predicted fields but no detected masks rather than totally empty return.
• peakdetect => scipy.signal.find_peaks:
  • peakdetect haven't maintain for a while, this line in peakdetect from scipy import fft, ifft only works when scipy < 1.4.0, it should changed to from scipy.fft import fft, ifft. after 1.4.0, better delete the dependence of peakdetect
• unused import 'psutil'
  • this package doesn't on dependence and also not used. should be delete

[yyang35]

@kevinjohncutler I am doing pytest on my end, which scan out some dependence error on omnipose. Can you take a look? ^-^