Don't wrap global phase if parameterized

[dieris]

Summary

Dynamical decoupling cannot currently be applied on a circuits where global phases contain unbound parameters, due to the _mod_2pi wrapping.

This PR replaces the _mod_2pi method with the global phase setter in DAGCircuit, wrapping the phase to the [0, 2π) interval (for floats).

[coveralls]

Pull Request Test Coverage Report for Build 5729356354

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 51.13%

---

Totals
Change from base Build 5728346612:	-0.03%
Covered Lines:	3304
Relevant Lines:	6462

---

💛 - Coveralls

[taalexander]

I believe this code was inherited from Qiskit when I started this PR. It seems ok if tests are passing but should be tested with hardware.

[dieris]

Yes, indeed, I believe qiskit dynamical_decoupling would have the same problem, I can open an issue there too