renamed Scalar to String
made string rep of unconverted value more readable in the context of a report
added missing imports of Date and Datetime. got rid of obsoleted from_text and to_text globals
added MappedString type which is behavior commonly used in text file handling
added has_unconverted() and unconverted_report() to the contain types along with a usage test scenario of an edi file to the main unittest which uses them.