Port code to use `pandas.DataFrame` objects

Add the following functions, which existing code to use
`pandas.DataFrame` objects:

 - `integrate_fft`: Integrate a bandwidth around a target frequency in
   DStat FFT results.
 - `dstat_to_fft_frame`: Compute FFT for set of DStat measurements.
 - `dstat_to_frame`: Convert DStat text file results to
   `pandas.DataFrame`.
 - `reduce_dstat_data`: Reduce measurements for multiple DStat
   experiments to a single row per experiment with an aggregate signal
   value.

See function docstrings for more details.