PerpAccount.base_token_value display issue

```
>>> sl.net_value
« InstrumentValue:        -0.00040000 BTC »
>>> sl.perp_account.base_token_value
« InstrumentValue:       400.00000000 BTC »
>>> sl.perp_account.base_position_raw
Decimal('400')
```

Instrument value of perp base token position is shown in raw. Should convert by decimal.