Change directory into package parent directory to support loading glade
UI files using relative paths.

Add support for all experiment types in `dstat_data_to_frame` function.

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.

Add `get_experment_data` method to 0MQ API, which returns a
`pandas.DataFrame` containing two columns, `time_s` and `current_amps`
which correspond to the *time in seconds* since the start of the DStat
experiment, and the measured current in *amps*, respectively.