Establish communication between pod operation and inverter control

[taesungh]

To operate the inverter from the main FSM, we need the pod operation program running on the main Raspberry Pi to communicate with the inverter control running on the Pi Pico. We can use serial communication over USB-to-USB and will need to establish a basic protocol/schema for the data (speed, amplitude).

• [Pod] Create abstraction for driving inverter board #35
• Add functionality to read and parse serial data from pod control inverter-control#24
• Add control message expiration time inverter-control#37

[taesungh]

Update: powers is changing the design where we will have individual Pi Picos for each of the three phases. Accordingly, we'll need to be able to indicate the phase offset as part of the communication protocol or through a hardware mechanism (e.g. short some control pin). Alternatively, we could load individually pre-programmed versions which include the phase offset.

Since the phases are no longer controlled by a single device, we may need additional synchronization mechanisms to ensure the three phases stay equidistant.

The main Raspberry Pi should have enough USB ports to support individual connections. Otherwise, we may need to look into wiring the UART pins between devices.