Changes to Execution Contexts based on signal

Closes #8
### Program Execution
* If currently selected volume goes out of range based on connection, no programs can be started.
* If a program is running and its volume goes out of range based on connection, execution is halted, but held.  If signal is re-acquired the program will continue execution.
* Programs running on a local volume will continue to run even if all signal is lost.

### Immediate Mode
* **Un-Manned** - If signal is lost, execution is halted but held.  If signal is re-acquired, the program will continue execution where it left off.