Greetings,

First of all, thank you for this great package!

The other day I needed to calculate sound pressure levels in specific frequency regions. I wanted to use the Signal.bandpass_frequencies method which needs Frequencies class as input frequencies. The code I used is the following:

import numpy as np
import matplotlib.pyplot as plt

sample_rate = 44100
signal = np.random.randn(10000)
s = Signal(signal, sample_rate)

freq_bands = np.array([[100, 200], [200, 400], [800, 1600], [3200, 6400]])
frequencies = Frequencies(center=freq_bands.mean(axis=1), lower=freq_bands[:, 0], upper=freq_bands[:, 1])

f, o = s.bandpass_frequencies(frequencies=frequencies)
Lp = o.leq()
fig, ax = plt.subplots()
ax.plot(f.center, Lp, ".-")

However, when using the following code, I get "TypeError: 'Frequencies' object is not subscriptable". I want to therefore propose a small change for the base Frequencies class by adding __getitem__ method, like in other child frequencies classes to resolve this issue and make Frequencies class usable also as stand alone.