Add label to input layers and more efficient circuit construction

[n28div]

This PR adds a fixed parameter to each input layer that acts as a label. Label can be a user specified object, it just needs to implement the __str__ method.
It is useful in logic circuits to assign to each variable the logic literal it refers to, but it can also be useful in other contexts to have a higher degree of interpretability of the circuit.

Labels have only been in the plotting function and in the construction of logic circuits, but it is relatively straightforward to do the same on other templates.

It also implements some enhancements to circuit construction, mainly caching pre-processing operations.