add bus voltage over limit

[asssaf]

• Describe the scope of your change--i.e. what the change does and what parts
  of the code were modified. This will help us understand any risks of integrating
  the code.

Add getBusVoltageOverLimit and setBusVoltageOverLimit functions to read & write the BOVL register.

• Describe any known limitations with your change. For example if the change
  doesn't apply to a supported platform of the library please mention it.

Only implemented for the BOVL register for now.

• Please run any tests or examples that can exercise your modified code. We
  strive to not break users of the code and running tests/examples helps with this
  process.

This doesn't affect existing tests/examples.
Example code:

  ina228.setBusVoltageOverLimit(3000.0);

  float limit = ina228.getBusVoltageOverLimit();
  Serial.print("Bus over voltage limit: ");
  Serial.print(limit);
  Serial.println(" mV.");

  uint16_t flags = ina228.alertFunctionFlags();
  if (flags & 16) {
    triggered = true;
    Serial.print("Triggered at ");
    Serial.print(limit);
    Serial.println(" mV.");
  }