adds B/2 term to Paasch element, fixes #251#256

mdmurbach wants to merge 3 commits intoECSHackWeek:mainECSHackWeek/impedance.py:mainfrom

mdmurbach:bug/fixes-equation-for-Paaschmdmurbach/impedance.py:bug/fixes-equation-for-PaaschCopy head branch name to clipboard