Added cumulative incidence to `summary_longformat.xlsx`
CHANGES:
- Added code in summary_longformat.pyto calculate cumulative incidence;
- Switched from openpyxltoxlsxwriterfor lower memory usage when writing to summary_longformat.xlsx.
CHANGES:
summary_longformat.py to calculate cumulative incidence;openpyxl to xlsxwriter for lower memory usage when writing
to summary_longformat.xlsx.