Begin using SIUnits

- Rename 's' -> 'sig' as the generic name for a Signal to allow for Seconds
- Re-export Second and its common short names
- Add a cursory check to ensure that time is in Seconds if it is unitful

Forcing all Signals to have time be AbstractVector{SIQuantity{T,0,0,1,0,0,0,0}}
seems a bit heavy-handed and cumbersome to me, but perhaps it would be
workable. I'll start with this for now.
Add a first-pass at some time restrictions