AP_RangeFinder: Maxbotix sonar I2C multi devices

[DomenicoPatella]

Add a backend driver for each maxbotix I2C rangefinder as different address.

Tested with two gy-us42 sonar , one of default address 0x70 and the second with 0x68 address

[rmackay9]

@DomenicoPatella. thanks for this. I guess though that during testing you needed to set the RNGFNDx_ADDR parameter even when using the default I2C address? I think we need a bit of code added to the MaxbotixI2C class that uses the default I2C address is the RNGFNDx_ADDR parameter is zero.

[DomenicoPatella]

I suggest to check the RNGFNDx_ADDR > 0 before to assign the driver

 if(params[instance].address){
        FOREACH_I2C(i) {
          ...

In this case only the device with a valid address will be initialized.
There is no way to change the address for Maxbotix sensor, in this case we can do it
Otherwise we can write a default address only if user leave zero on address parameter

[peterbarker]

This has been in a slightly different way in master which preserves existing behaviour (as @DomenicoPatella suggested, actually).

Thanks for this PR!