TSVSerialiser: Add support for numbered sections

There is exactly one case where this is needed:
`<AxisFormula 1>...</AxisFormula>`

TSVSerialiser will now parse these sections and save them with
the space and number included in the section name.

Refs #9970
Add support for loading <AxisFormula n> sections

Refs #9970
Merge branch 'feature/9970_refactor_project_serialisation' into develop