Addition of a ParametrizedFusedGate class

[renatomello]

This issue / request appeared at least twice recently: once when @BrunoLiegiBastonLiegi was implementing the new openqasm parser, and also when people from Quantum Algorithms @ TII were trying to create custom gates using the current FusedGate class.
We would benefit from a FusedGate object that keeps track of the parameters of parametrized gates that were fused.