Make the IMU implementation more generic so it can be tested.

[cal-pratt]

This PR adds the AltIMU to the Rov class. It also fixes #168 by reading the AltIMU (and voltage and current) sensors on their own Observable.

Details:
In order to test the Rov class, the AltIMU must be able to mocked. The mocked class must not rely on I2C and needs to have some interface which can be implemented by the test code. Following a similar strategy to the Maestro code, the Rov requests a generic IMU which implements 5 sensor interfaces.

This change removes the dependency between the AltIMU and rx observables by only exposing simple get methods (linking these to an rx observable can be done from the rov class itself).

Future work will be creating proper tests in RovTest and RovSimulator classes

[cal-pratt]

Update

I reworked the IMU class such that it can be tested. The I2C bus passed to the IMU is now a list of I2C devices. This allows us to create a mock set of i2c devices that can be used to test the IMU reading/setting registers. the I2C class has also been made thread safe in the event that we want to read one sensor more than another. (ie, temperature could probably be slower than gyro data). RovTest and RovSimulator still need proper tests defined, however, the IMU itself now has adequate test coverage! Should be good to merge once it gets tested on the actual ROV

[ConnorWhalen]

LooksGoodToMe, just test it on the ROV as you mentioned