spectrum/core.py: add ifft method

Created by: cpankow

Fixes #234 (closed) . I've tested that this returns almost what we want with two issues:

• I'm having trouble setting the sample rate from df, so I set dx instead, but the units come out as inverse Hz. Not ideal.
• I'm struggling to understand the normalization convention used in TimeSeries.fft, so the output of ifft is not currently normalized correctly.