Make attribute validation sensitive to FHIR version

[ewoutkramer]

So far, .NET attribute validation is not aware of the Since attribute. This was never a problem, as all attributes that have Since do not take part in validation, they just provide metadata.

This needs to change (and was already not completely true):

• NotMapped should validate whether an element still appears in a FHIR version and then report an error.
• FhirElement can repeat and should validate whether an element is valid for a given FHIR version (not validated at this moment)
• AllowedTypes has been promoted to a versioned attribute, since the set of allowed types can vary per version.

We should probably extend the ValidationContext with a property (as we've done before, the pattern exists) that represents the applicable FHIR version (or highest if let out).