Voltage and Current element fixes

[cwhanse]

• removes duplicate "VoltageOpenCircuit " element
• removes 'VoltageMaxPower' redundant with 'VoltageAtMaximumPower'. Retains 'VoltageAtMaximumPower' because the element is superclassed from TaxonomyElementNumber rather than TaxonomyElementString.
• removes 'CurrentMaxPower' redundant with 'CurrentAtMaximumPower'. Retains 'CurrentAtMaximumPower' because the element is superclassed from TaxonomyElementNumber rather than TaxonomyElementString.
• remakes 'CurrentShortCircuit' to be superclassed from TaxonomyElementNumber rather than TaxonomyElementString.