Fix upper limit extraction from grid based posterior
The test suite fails with CWInPy 1.2.0 and pesummary 1.2.0 (see, e.g., https://git.ligo.org/computing/sccb/-/issues/1441#note_948078). This is because the upper limit when calculating a posterior on a grid gets returned as a numpy array rather than as a single float value. This MR fixes this.