Bug with Temporal Monitor when running in parallel

Summary

@muralikrishnan pointed out that he was having this error:

When a temporal monitor is used, and you run on multiple cores, you get the following error:

Error{1}>                                                                                                                                                                                                          
Error{1}> *** Error:                                                                                                                                                                                               
Error{1}>     Internal OPAL error:                                                                                                                                                                                 
Error{1}>     basic_string::_M_construct null not valid                                                                                                                                                            
Error{1}>     basic_string::_M_construct null not valid   

This happens for example running test_fft.in, only when run on multiple cores. When running on a single core it works fine.

The problem seems to come from the line auto stats = lossDs_m->computeStatistics(1); (https://gitlab.psi.ch/OPAL/src/-/blob/master/src/Classic/AbsBeamline/Monitor.cpp#L132), which was added in !557 (merged)

I don't exactly know why this line was added or why it gives an error here. Maybe @kraus can comment.