Decide on a naming convention

[znicholls]

Naming is tricky, but as we move towards auto-generated code we will want to decide on something e.g. do modules always start with m_ or a more specific package-specific prefix (which is what example ftoml does, all modules start with ftoml_ and they have top-level module which imports key parts of the API to make things easier for users), do name modules so they 'match' the derived type they contain e.g. m_error_v defines ErrorV.