Renamed variables, classes, methods for clarity.

Renamed the following:
1. base.OrderedPart.__part as __attributes;
2. base.OrderedPart.__part_attributes() as __fields();
3. base.Attribute as Field;
4. exporter in base.Attribute.export() as format;
5. base.PartSchema.attributes() as fields();
6. attributes in base.PartSchema.part() as fields;
7. base.Format.property() as attribute();
8. formats.TextFormat.property() as attribute();
9. formats.XmlFormat.property() as attribute();
10. tests.bom_cost.part_attributes as part_fields.