Functions to measure phase points

[mingzeng8]

Add functions to measure phase points of axial E_z field.

[RemiLehe]

Thanks for this pull request! However, at this point I am not sure that I fully understand the purpose of the changes. Could you edit the description above in order to explain the new functionality in more details?

Also, note that pyflakes raised an error here. Could you fix the issue and update the pull request? (You can git push to the same branch and this will automatically update the pull request)

Thanks a lot!

[mingzeng8]

Thanks for your review. I accidentally worked on an older branch. Now I moved everything to the latest dev branch and it works well. But I'm not sure what are the failures in the checks. Actually I ran the same check on my local machine and it has passed.