Differentiate between indices, lengths and other values in model

[treiher]

When parsing conditions and location expressions in a message type we should differentiate between fields which are used as index (indicated by a First attribute), length (indicated by a Length attribute) or other value. This information is needed to be able to ensure that lengths can be added among each other and can be added to an index in the generated code. Ada does not allow the addition between different types, thus an appropriate conversion is needed.

[senier]

Alternatively (and more cleanly) a custom "+" operator can be implemented that allows for addition of index and length types. This way, we errors where indices are added in an erroneous way.

[treiher]

Rethinking the issue I cannot imagine a use case where it makes sense to treat a field as an index. So we should only differentiate between length fields and other fields.