DOC: Time-Series Analysis Tutorial

[rstoneback]

Is your feature request related to a problem? Please describe.
Provide some background on how pysat data can be used for time series analysis.

Describe the solution you'd like
A basic tutorial.

Describe alternatives you've considered
Do nothing.
Alternatively, point people towards the specific data type documentation (pandas, xarray)

Additional context
Was listed as pending in documentation.

[aburrell]

I like the idea of showing how people can select a limited number of points by time for an xarray and a panadas instrument, and then pointing them to the xarray/pandas docs.

[rstoneback]

Thanks for the suggestion. Cheers.