Add date range validations

[fjwong]

Closes #107

• Adds support for minValue and maxValue extensions in date, time and dateTime items.
• Adds support for dynamic calculation of min/max time ranges via cqf-expression extensions.
• Updates SDC demo Questionnaire with examples of static and dynamic range validations.
• Updates SDC capabilities document.
• Adjusts min/maxValue validation messages to support both numbers and dates/times/datetimes.

Notes

• According to the SDC docs, dynamic values for min/maxValue extensions can be done with the cqf-calculatedValue extension.
• Here we implement them with cqf-expression instead.
  • While cqf-expression doesn't explicitly state minValue and maxValue as recommended locations to use it, it does mention it is possible to use in other places.
  • I also believe the reference to cqf-calculatedValue in the min/maxValue docs may be mistaken, since the extension doesn't allow expressions like the ones mentioned in the docs, and only the name to some variable declared in an external library defined in a cqf-library extension.

[fjwong]

@hanny-ph Thanks a lot for the assistance in the review. :)