Suppress zero div warning in curve analysis (#1098)

### Summary

This PR suppresses runtime warning for zero dive error in curve
analysis. Such data points are safely managed by LMFIT so user doesn't
need to take care of it. This PR makes standard error cleaner.

Fix #1087 

### Details and comments

---------

Co-authored-by: Helena Zhang <Helena.Zhang@ibm.com>

qiskit_experiments/curve_analysis/curve_analysis.py

@@ -322,10 +322,15 @@ def _objective(_params):
             ys = []
             for model in models:
                 sub_data = curve_data.get_subset_of(model._name)
+                with np.errstate(divide="ignore"):
+                    # Ignore numpy runtime warning.
+                    # Zero y_err point introduces infinite weight,
+                    # but this should be managed by LMFIT.
+                    weights = 1.0 / sub_data.y_err if valid_uncertainty else None
                 yi = model._residual(
                     params=_params,
                     data=sub_data.y,
-                    weights=1.0 / sub_data.y_err if valid_uncertainty else None,
+                    weights=weights,
                     x=sub_data.x,
                 )
                 ys.append(yi)

releasenotes/notes/suppress-runtime-warning-a741dc96f6a0ce7a.yaml

@@ -0,0 +1,8 @@
+---
+other:
+  - |
+    NumPy runtime warning for zero division is suppressed in :class:`.CurveAnalysis`.
+    This warning could occur in the edge case where the experiment data
+    may contain data point with zero uncertainty.
+    Such data point is safely ignored by LMFIT, since it may apply infinite fit weight.
+    This runtime warning suppression makes standard error cleaner.