Add help for Fscan programs

Detailed Description

This MR adds some help description text for what these Fscan programs do: they take in SFT files and produce Fscan data products as outputs - ASCII files of spectra (normalized or not), spectrograms, persistency figure of merit, and coherence between two channels.

Closes #607 (closed)

API Changes

Please tick one of the following options:

• These changes do not modify the API.
• These changes do modify the API, and are backwards compatible.
• These changes do modify the API, and are backwards incompatible.

For examples of changes that do not modify the API and/or are considered backwards (in)compatible, please see the contributing guide.

Justification for Backwards Incompatible Changes

Review Status

@david-keitel or @karl-wette may wish to have a look to see if this is sufficient or more is needed