Experimental multiple i2c bus support

[Gadgetoid]

This is an experimental fix for #33 (I have no hardware to test locally, so I've only tested that this compiles).

It fixes the issue of the i2c read/write and multi functions being global, so that if you attempted to initialise several sensors on different i2c busses then you'd only actually be talking to the last initialised bus, since the read/write/multi function pointers were being overwritten.

This is achieved by moving the i2c IO functions into VL53L1_DEV and passing them in through _TOF_LIBRARY.initialise instead of _TOF_LIBRARY.VL53L1_set_i2c.

This should have no impact on existing setups.

It might be worth also moving the i2c mutex into VL53L1_DEV, since it only needs to be per i2c bus.

[nav2u]

It looks like this code update fixes the issue. I get five individual distance readings now with five sensors, which I confirmed by blocking each sensor individually. Thank you!

[Gadgetoid]

Wow. I'm actually a little surprised it hasn't subtly broken. Thanks for reporting back.

[nav2u]

No problems, thank you for taking the time to help out 👍

I will be messing with these sensors for the next few weeks, so you'll probably hear more from me on here :)

[DmitryBrown]

Would love some help on this! I cloned the multiple-i2c-bus-support branch. I also set up two additional physical i2c busses by retagging 4 gpio pins. However i'm still running into issues described - one sensor reading is applied to all three.