Initial stage of @kevingill1966 PR #4

This is part of PR #4 squashed into a single commit and based on the feature/python3-support branch.

It includes:
1) Correct MSH field numbering
2) Handle further nesting in the fields (subfields, repetitions)

It does not insert a dummy initial element in sequences to fix numbering, so numbering is still 0 based for now. It also removes the special accessor support for now. Numbering and accessors will be added back in a separate PR once this is merged.