Quick spectral analysis plots

Create quick spectral analysis plots.

Inputs:
data - the data for spectral analysis
K - averaging parameter, explained below
beta - beta value for the Kaiser window
out_form - spectral analysis option
samp_freq - data sampling frequency

Outputs:
out - calculated spectral estimate in form specified by out_form

Averaging Parameter:
The spectra are smoothed by averaging over windows 1/K times the full
length of data. This smoothing method is based upon a technique described
in section 13.4 of "Numerical Recipies in C++", Second Edition.

Windowing:
Each of the above windows are multiplied with a Kaiser window with the
provided window beta value given by the beta input parameter.

Output Format Options:
'as' - amplitude spectrum
'ps' - power spectrum
'asd' - amplitude spectral density
'psd' - power spectral density
'rms' - integrated rms motion