Wip/min thresholds

[scottleibrand]

Add sanity checks to prevent min_bg and max_bg from being set too low, or min_bg being set too high. (We can safely allow max_bg to be too high: openaps just won't do anything.)

[bewest]

We should also check that the if (bgtargera.units !== "mg/dL") { throw "bad units, we only do mg/dL"; } or something.

[scottleibrand]

@oskarpearson please test the unit check with both mmol/L and mg/dL units. if it fails properly in the mmol/L case and works normally with mg/dL, I think we can merge.

[bewest]

I've fixed some of the problems in the last set of patches in #39, would like to get this into dev.