Add new prefixes for ampere, henry, ohm and volt

New prefiexes and implicit conversions for:
 * ampere: nA, µA, mA
 * henry: nH, µH, mH
 * ohm: nΩ, µΩ, mΩ, kΩ, MΩ
 * volt: nV, µV, mV, kV