Inconsistent use of 'messages' vs 'elements' to describe Phenopacket building blocks

[azankl]

Phenopackets' building blocks are sometimes referred to as ‘messages’ and sometimes as ‘elements’ in the docs. For example, ‘Individual’ is called an element (here), while ‘File’ is called a message (here). I think ‘element’ is better, more in keeping with the ‘building block’ analogy, ‘message’ is more protobuf terminology.