Unit u_A interpreted as attoamp

```python
from PySpice.Spice.Netlist import Circuit
from PySpice.Unit import *

circuit = Circuit('Title')
circuit.I(1, circuit.gnd, 'n1', 2@u_A)
circuit.R(1, 'n1', circuit.gnd, 1@u_Ω)

simulator = circuit.simulator()
analysis = simulator.operating_point()

print(analysis['n1'][0])
```
yields `2e-18 V` (two attovolts) instead of `2 V`. 

Ngspice version 47

Workaround: `2000@u_mA`