Refactor/new structure

[davidesner]

• Backward compatibility for legacy manifests on input as well as output
  • this means that it is possible to have empty columns and filled primary key, in which case the manifest will be always forced to legacy.
  • Fixed BaseType class type and added convenience methods
  • has_headers configurable from interface
  • Fixed tests, split to more so it's "unit" test
  • Remove columns as named attribute from every api, keeping it in kwargs for backward compatibility
  • Removed calls to legacy methods internally.
  • Added tests for original attribute order compatibility + replacing columns with schema, allowing for list of strings