Add vignette on filtering on time periods

[rmbielby]

Describe the solution you'd like

We need a good vignette to explain the subtleties / possibilities around selecting time periods using eesyapi:

• How to select a simple combination of possible time periods
• How to select all time periods before a given time period
• How to select all time periods after a given time period
• How to select all time periods between two given time periods

I'm thinking this should be linked appropriately from the workflow vignette.

Additional context

Some of this isn't implemented yet, the work is outlined in #44

Linked equivalents for geographies (#43 ) and filter items (#45).