Create FrequencyDomain results class

Currently, the results of the frequency_domain function are stored in a Python built-in dictionary.

It would be a nice improvement if the results were stored in a custom dict-like class with additional useful methods, such as plot.

For instance, a plot method could depict the power spectral density (PSD) chart like the following:

freq_domain_results = frequency_domain(rri, ...)

freq_domain_results["lf"]
# 134.56
freq_domain_results.plot()

There is something similar already created for time varying analyses:

rhenanbartels / hrv