Adding Delay, ShiftPhase, and SetPhase instruction handling to pulse simulator

[DanPuzzuoli]

Summary

This handles the first part of issue #736 , adding support/testing for the Delay, ShiftPhase, and SetPhase instructions.

This PR depends on the changes in PR #811.

Details and comments

• Delay didn't require any additional handling, as it doesn't enter the qobj.
• A new test has been added to ensure delays are properly handled.
• Handling of phases in the function experiment_to_structs in digest_pulse_qobj.py and the function chan_value in numeric_integrator.cpp has been changed from relative to absolute.
  • Previously the data structure storing phase changes stored relative changes, as the FrameChange instruction (which has been replaced by ShiftPhase) was relative to the current phase. However, SetPhase is an absolute instruction, and therefore requires absolute phase handling.
  • To handle the above point, experiment_to_structs has been changed to construct an array tracking absolute phases, and chan_value has been changed to look this value up (whereas previously it would add up all the relative phase changes).
  • Handling of the SetPhase instruction has been added to experiment_to_structs.
• Several tests for SetPhase and ShiftPhase have been added.

[vvilpas]

LGTM!