Change Ammeter and graph current units

[marlitas]

We want to switch to microamps instead of milliamps and additionally have a mode where a less than sign appears when the current is below the detection threshold.

[brettfiedler]

After adjusting the quantum efficiency constant to 1.0 (all photons interact with an electron, though not every photon has enough energy to eject deeply buried electrons), the range of possible currents is from max 1600 uA down to many values below 0.1 and 0.01 uA.

Need to determine if we need precision down to 0.1/0.01 uA or if we can get away with saying < 1 uA for everything below that precision. Need to make sure that scale is not too wide to confuse when we've reached low current with the visuals.

[brettfiedler]

Need to investigate/talk to stakeholders and finalize decision before handing to devs.