working on S4 version
unit tests for S4
updated documentation
draft methods output file
inst/tests/S4_write.R not passing
Progress! Working write_eml methods now, mostly working read_eml methods
The write_eml approach uses a custom function in place of coercions, and probably should be written as a method instead.
The read_eml approach modifies XML::xmlToS4 to handle ListOf option.  Both need tests written still