Update documentation v.2.1

[EmilieSkog]

In ModelingGuide_Measurement and ModelingGuide_MeterReading we need to add details on:

• When to use Measurement vs MeterReading
• How to use Measurement/MeterReading
• Validation of whether the listed scenarios are included or not

For ModelingGuide_Measurement we should also document the decision made wrt Issue #76.