Chat: improve takeoff and fly-to commands

[rmackay9]

This PR includes two small enhancements to the chat module:

1. Instructions to the assistant have been updated to increase the chance that it will set the "frame" field correctly.
2. Add a new get_location_plus_offset function to help the assistant more quickly and easily respond to commands like, "fly 100m North". Without this the assistant tries to work out the calculation itself each time and this leads to some failures

Both these changes have been tested and work. Below are some screen shots from a recent test.

I should add that during testing I saw the chat window apparently freeze up twice probably waiting for responses from the Assistant that never arrived. I haven't seen these freezes before but I assume they are unrelated to these changes and have added this to the issues list.

[rmackay9]

Thanks!