Make Baseline & Period guidance Musts

At 2.03 three should statements (of which originally two were musts) should be made into must statements at version 3.0.0.

The rules to be added are:

• The @value must be omitted for qualitative measures
• The @value must be included for non-qualitative measures
• The @value must be a valid number for all non-qualitative measures

And they are found in three places:

• iati-activities/iati-activity/result/indicator/baseline/@value
• iati-activities/iati-activity/result/indicator/period/target/@value
• iati-activities/iati-activity/result/indicator/period/actual/@value

This helps ensure the quality of data being reported, and will make it easier for data users to analyse results data.