Refactor siggen FMCW

[mbr0wn]

• No functional changes
• Attributes are properly const and/or private
• Updated some comments

[mbr0wn]

This one is not strictly necessary... but it does make use of a more modern C++ style.

[mbr0wn]

And note it actually does not change the behaviour of said block at at all.

[stwunsch]

Thanks!