Map - commented code for costing allowing other types of variable in the TimeSlider

[bobular]

Works towards #761

• comments relating to allowing other continuous variables (number and integer)
• comments relating to allowing string ordinals and decide on approach to take

Allowing the other continuous variables doesn't look too tricky.

It's looking like string ordinals (for example with 'YYYY-MM' values) may require a separate TimeSlider component for the following reasons

• ordinals are filtered by checkboxes so despite being shown in date order, they could have gaps (e.g. no December)
• therefore range selection and (in future) stepping forwards and backwards are going to be tricky
  • for example, the onChange callback of Brush will return a list of selected values (in domain.values) instead of the range (domain.x0 to domain.x1)
• I'd (@bobular) suggest disabling all kinds of selection in the ordinal component (e.g. no Brush) - at least as a Phase 1.
  • later on it might be decided that a different UI is needed (e.g. discontinuous selections similar to checkboxes)
  • we will need appropriate user feedback when users try to make a selection to explain why they can't