Support 'split_by' as numeric

This Pull Request includes the extension of the 'split_by' parameter, to support splitting for section level higher than 'section' (issue #1347). The 'split_by' parameter can be specified as [X] (or [X]+number) such as the report is split into separate html files by level [X] of sectioning (without/with numbering) For example, the report can be split for a subsection, i.e. a header 1.1.1 if split_level = 3.

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