text: Title: [Feature Request] Merge profile sessions from multiple runs to get statistical numbers like Average/p95/mean
  Body: Hi,
  
  Firstly I want to thank you for having this tool and I have benefited a lot from it to optimize our codebase!
  
  A feature that has been in my mind for some time: **since in real case, different modules in code may have different time variances and to get more accurate profiling results, we need to run multiple times and look at average/mean/p95 numbers for further optimization.** 
  
  Currently I don't see an option to do that in pyinstrument and it would be best if it can support merging multiple profile sessions from multiple runs to get statistical numbers like Average/p95/Mean.
  
  Thank you!