Add option to encode/decode EXI files

It is rather cumbersome to encode/decode schema aware EXI files as they need access to the Daffodil XML resolver in order to import/include schemas correctly. Having a simple:

"daffodil encodeEXI -s /path/to/schema /path/to/file.xml"

to convert an XML file to an EXI file will be very helpful for testing. A decodeEXI command would also be added.