[gx1400]

Closes #281

AnsrvrMessage recognises ANSRVR's group traffic in a received message. Both kinds read N:GROUP text; who sent it tells them apart:

• Relay: another member's post, sent in their name by ANSRVR's host (APWW11, qAO) with no message ID. Gives the group, the poster and the text.
• Notice: ANSRVR's own reply about a group, such as N:NEDR 2 Messages Sent or N:C4FM Now Monitoring with 2 Members.
• Neither: a message with a message ID from any other station (a person typed it), ANSRVR's replies that name no group (No Memberships, Duplicate (retry?) request suppressed), and lower-case n:.

TryParse takes the IncomingMessage from MessageReceived, or the sender, text and message ID. A relay gated to RF by a two-way IGate is recognised too, once MessagingStation unwraps it. The group is kept as written.

The tests use the lines captured on APRS-IS in #272 and #281. The Messaging README and the messaging design note cover it.

AprsMap can replace its SHIM: G34 recogniser (APRS-NET/AprsMap#442) with this once it ships.