Changed to explicitly define all methods.

Changed
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* Refactored ModelDataFields class to explicitly define all methods
  rather than generating them dynamically from ScalarDataFields.
  Downside is that it doesn't obey DRY, but the upside is it should be
  more clear to a new reader.
Removed now unused functions.

Removed
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* Removed unused functions that dynamically generated method for
  ModelDataFields class.
Removed unit tests for non-existant functions.
Merge pull request #36 from mcflugen/refactor-field

Refactor ModelDataFields class