Read and set every bias channel attribute, with labels

[mikelangmayr]

• Only the voltage was reachable, though each channel also carries a label, a power-on order, and on some banks an enable and a current limit
• An XVBias board was rejected outright as "not a bias board", so its voltages could not be read or set at all
• Describes the banks as a table per module type, giving each its prefix, channel span, voltage range and which attributes it has, replacing a switch that hardcoded prefixes and ranges
• bias <mod> <chan> now reports every attribute the bank carries, and bias list [<mod>] reports every configured channel
• Fields can be set together, bias 10 25 V 3.5 IL 20 ORDER 4, applying the module once where separate commands applied it once each
• Every read and set names the channel label, so a voltage is never shown without saying which bias it is
• Asking for a field the bank lacks is refused rather than writing a key the board ignores, and the current limit is capped at 250 mA
• Adds an emulator system file declaring a bias board, since the existing one declares none and no bias call could be tested
• Verified on the emulator and against an Archon, reading all three bank shapes, HVLC, HVHC and the two XVBias polarities

$ bias list 10
MOD10 chan 1 LVLC V=2.3 ORDER=2 LABEL="Bias Gate"
MOD10 chan 2 LVLC V=3.3 ORDER=2 LABEL="Bias Power"
MOD10 chan 3 LVLC V=0.55 ORDER=2 LABEL="Diode Sub"
...
MOD10 chan 25 LVHC V=3.3 ORDER=3 ENABLE=1 IL=10 LABEL="PullUp"
MOD10 chan 26 LVHC V=0.0 ORDER=1 ENABLE=0 IL=50 LABEL="Misc 1"