Corrected computation of HWP angle and bug in computation of polarimetric efficiency + various minor improvements to log file and documentation

• Clarified meaning of standard deviation of fitted center coordinates in log file
• Very minor correction to computation of HWP angle from headers
• Corrected minor bug when computing polarimetric efficiency when the number of Q and U cycles is unequal
• Added warning when spread of star polarization per HWP cycle is large and suggest to try normalized double difference