CircuitSymbols labels should always be rendered as text in Latex

[goerz]

>>> latex(CircuitSymbol('BS', cdim=2))
'BS'
>>> latex(CircuitSymbol('BSBS', cdim=2))
'\\text{BSBS}'

It would be better not to make a distinction between "short" and "long" labels, but always set the labels in a \text