Expose HalfAngleCal (#1091)

### Summary

The Half Angle Calibration experiment was not exposed. This PR adds the
necessary imports to correct this.

Author: TsafrirA

qiskit_experiments/library/__init__.py

@@ -131,6 +131,7 @@ class instance to manage parameters and pulse schedules.
     ~calibration.FineAmplitudeCal
     ~calibration.FineXAmplitudeCal
     ~calibration.FineSXAmplitudeCal
+    ~calibration.HalfAngleCal
     ~calibration.RoughAmplitudeCal
     ~calibration.RoughXSXAmplitudeCal
     ~calibration.EFRoughXSXAmplitudeCal
@@ -150,6 +151,7 @@ class instance to manage parameters and pulse schedules.
     RoughFrequencyCal,
     FrequencyCal,
     FineFrequencyCal,
+    HalfAngleCal,
 )
 from .characterization import (
     T1,

qiskit_experiments/library/calibration/__init__.py

@@ -49,6 +49,7 @@
     FineAmplitudeCal
     FineXAmplitudeCal
     FineSXAmplitudeCal
+    HalfAngleCal
     RoughAmplitudeCal
     RoughXSXAmplitudeCal
     EFRoughXSXAmplitudeCal
@@ -66,3 +67,4 @@
 from .fine_drag_cal import FineDragCal, FineXDragCal, FineSXDragCal
 from .frequency_cal import FrequencyCal
 from .fine_frequency_cal import FineFrequencyCal
+from .half_angle_cal import HalfAngleCal