PCB: use up to 3 zener diodes

[ThomasWaldmann]

related to #14 (and see also #86), but also good for more flexibility:

if a new pcb revision is made, we could have 3 places for zener diodes, so we can have:

• 200V ZD + 200V ZD + wire == 400V (as for Si22G, SBM20, SBM19 gm tubes)
• 200V ZD + 200V ZD + 100V ZD == 500V (as for LND712 gm tube)
• any other combo of 2 or 3 ZDs depending on availability of specific ZD voltages
• any other combo of 2 or 3 ZDs depending on the voltage requirement of the gm tube

[ThomasWaldmann]

note: it is hard to get zener diodes with more than 200V.