redfast00

`core:freq_lower_edge` and `core:freq_upper_edge` aren't mandatory (per https://sigmf.org/#subsec:AnnotationsArray); it would be nice if annotations that don't have them could also be displayed

I built a display that shows when the next train to a certain station is.  (The display shows that the next train to FSD (Oostende) is at 16:39 on...