Add notebook for FM broadband Sv/TS spectra processing

[LOCEANlloydizard]

With FM processing dev ongoing, we should add an example notebook showing the FM workflow for:

• band-averaged Sv
• Sv(f) spectrum computation
• TS(f) spectrum computation

Ref: echostack-org/echopype#1034

[LOCEANlloydizard]

#106 shows how to use full broadband TS(f)