Function templates for outputting CSV cells and rows.
Create ReportWriter class.

To serve as base of report writing class hierarchy, which will
replace print_stints_report() as better-structured vehicle for managing
report generation.
Create ListReportWriter and CsvListReportWriter classes.

Also place some common logic re. production of rounded Interval durations
in hours, into ReportWriter class.
Create SummaryReportWriter class.
Further work on ReportWriter and SummaryReportWriter.
In ReportWriter, a reference to the stints is now stored as member variable.
Further work on ReportWriter.
Nomenclature changes in ReportWriter.

Change do_write() becomes do_process_stint(), etc..
Improvements in SummaryReportWriter.
Create CsvSummaryReportWriter.
Adapt ReportingCommand to make use of ReportWriter-derived classes.

CSV option ("-c") now added to ReportingCommand.
Document "CSV" option in README.rst.
Various improvements to ReportingCommand and report writers.
Create HumanListReportWriter.
Create HumanSummaryReportWriter.

Some edits also made to other report writing code.