Make use of Unicore output delay header field

[jclark]

Headers in the UM980 protocol have an Output delay field which is documented giving the "Difference between the data output
time and GNSS signal receiving time" in ms. We should make use of this. It can enable a better estimation of time based on serial information alone, and better correlation with the pulse.

[jclark]

Relates to #77

[jclark]

We can potentially combine this with #84.

We add a delay field to TimeMsg, with type duration. Semantics are that the difference between the time the message was received and the time in the message is at least this.

• with no additional information, it defaults to zero
• with Unicore, we compute it as this delay field
• with u-blox for a nav class message, we compute it as the difference between the received time of TimeMsg and the received time of the first message in the same navigation epoch