Support adding annotations

[GassiusODude]

It would be nice to support user added annotations. Provided a user specified time and frequency selection, allow user to

• Create a SigMF meta file (archive) to point to the data file being analyzed
• add the annotation to a SigMF metafile
  • User provides a label
  • User provides a comment (i.e. "Unknown signal" or if they do know what the signal is, it can be more specific)
  • Time and frequency information is pulled from the UI's time/frequency selection.

I imagine using this as a manual step to identify burst of signal of interest for further analysis. Then the user can use these blocks to pull the signal for their own library of analysis (classification, estimation, or processing).