Low-level commands should no longer be called directly

This release completes the separation between low and high-level commands in the package. Errors from low-level commands called directly can not be handled gracefully. Calling these functions with Tcl functions gives us all the benefits of Tcl error handling.

Fixed serial_number_list return for no devices

The serial_number_list function now returns an empty list if no devices were found.

clear_queue now returns {0 elapsed_ms} on success

This release adds the high-level clear_queue function to clear an ADU100's transmit buffer. Successful calls will return a list of {0 elapsed_ms}, where elapsed_ms is the time it took to clear the buffer.

Added convenience Tcl functions

Added the commands

1. serial_number_list -- Return a list of connected serial numbers
2. send_command -- Send an ASCII command
3. query -- Send and ASCII command and return the response
4. clear_queue -- Clear the hardware's transmit queue