Add readme and documentation examples to testing.

[jankoslavic]

This should be done to prevent example breaking at upgrades

[ParNaderi]

Thanks for updates. So far I checked the following for Spectral data and they work fine.
input = 'GUI' - PSD is provided by user via GUI (graphically and tabulary)
input = (PSD, freq) - tuple of PSD and frequency vector is provided.
input = (x, dt) - tuple of time history and sampling period is provided.

I tried to check  "test_basic.py" getting the error of not finding folder "data". You might want to address correctly to folder "data". I copied "data" folder inside folder "tests" and it worked fine.

One last comment, in GUI, the default upper x and y axis values are very high. If a user inputs low psd at low frequency, it is hard to be visualized in the graph. You might want to give user to adjust the axis upper value.