[VN300] Moved sendStringCommand and recvStringCommand inside VNCommonSerial.

src/shared/sensors/Vectornav/VN300/VN300.cpp

src/shared/sensors/Vectornav/VN300/VN300.h

src/shared/sensors/Vectornav/VNCommonSerial.cpp

src/shared/sensors/Vectornav/VNCommonSerial.h

src/tests/sensors/test-vn300.cpp