Handle CSV export of the datetime multi-series with variable X-axes

[hrynko]

This change introduces CSV export of the datetime series with unequal X-axes. I know it is non-trivial to export data as CSV in the case of multi-series with variable X values, but it is still possible, at least for datetime series. It's an important and long-awaited input (given the issue referred below), and it also seems quite reasonable, since such data is seamlessly displayed on the timeline, thus should be available for export even with empty cells.

Fixes #1568

Type of change:

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes