manual fields not accessible by dict-like methods.

[YooSunYoung]

Related to #76

For now, dict-like methods, keys, values, items, __getitem__, __setitem__ and setdefault
find fields with Dataset.fields method, which don't have manually generated fields, such as size and numberOfFiles.

They should also be accessible with these dict-like methods.

One suggested solution is to add a flag in the DatasetBase.Field class i.e. manual : bool which is set as False by default.