Sending messages is broken

[jonofmac]

Currently vpw_analyzer does not have the ability to send messages.

This is primarily due to the fact that the ELM327 command set does not allow a graceful way to send a message without risking losing a message incoming.

The basic process to send a message while datalogging with a ELM327 device is this:

1. Configure device
2. Enter AT MA (monitor all) mode to see all bus traffic (vpw_analyzer's primary mode)
3. Cancel AT MA mode
4. Configure desired headers
5. Send message
6. Device waits some amount of time to see if it gets a response to the specified headers (ignoring other traffic)
7. Enter AT MA mode to continue seeing messages

Steps 3-6 mean that there's a window of time that messages can (and most probably will) be dropped. This is an inherent weakness with the standard ELM327 commands.

Some more advanced devices (OBDX VT Pro) offer additional modes to allow transmission while receiving messages. The decision needs to be made to support both modes or only the safer OBDX modes.