[Quantum] Add symbolic Pi representation

[lschuetze]

We should provide a symbolic representation of pi such that we can handle pi / 2 etc. as special cases.

[lschuetze]

The frontend should also try to factor pi from given angles in the QASM code. Unfortunately, the Qiskit QASM parser folds expressions already.

[lschuetze]

Question: Is there a better interpretation to this expressions?

Extracted from QASMBench/small/basis_change_n3/basis_change_n3.qasm:

u3(pi*0.5,0,pi*0.0564006755) q[2];
u3(pi*0.5,pi*1.5,pi*0.2945501109) q[1];
u3(pi*0.5,pi*1.5,pi*1.5) q[0];
cz q[1],q[2];
u3(pi*0.1242949803,0,0) q[2];
u3(pi*0.1242949803,pi*0.5,pi*1.5) q[1];
cz q[1],q[2];
u3(pi*0.0298311566,pi*1.5,pi*0.5) q[2];
u3(pi*0.7273849664,pi*1.5,pi*1.0) q[1];
cz q[0],q[1];
u3(pi*0.328242091,0,0) q[1];
u3(pi*0.328242091,pi*0.5,pi*1.5) q[0];
cz q[0],q[1];
u3(pi*0.1374475291,pi*2.0,pi*1.5) q[1];
u3(pi*0.9766098537,0,0) q[0];
cz q[1],q[2];
u3(pi*0.2326621647,0,0) q[2];
u3(pi*0.2326621647,pi*0.5,pi*1.5) q[1];
cz q[1],q[2];
u3(pi*0.5780153762,pi*0.5,pi*0.5) q[2];
u3(pi*0.6257049652,pi*0.5,0) q[1];
cz q[0],q[1];
u3(pi*0.328242091,0,0) q[1];
u3(pi*0.328242091,pi*0.5,pi*1.5) q[0];
cz q[0],q[1];
u3(pi*0.6817377913,0,pi*0.5) q[1];
u3(pi*0.5,pi*0.3593182384,pi*1.5) q[0];
cz q[1],q[2];
u3(pi*0.1242949803,0,0) q[2];
u3(pi*0.1242949803,pi*0.5,pi*1.5) q[1];
cz q[1],q[2];
u3(pi*0.5,pi*1.3937948052,0) q[2];
u3(pi*0.5,pi*1.1556453697,pi*0.5) q[1];