Compare file dates on .mrc and .xml (and .xml and .rdf) to decide on 'skip'

[dazza-codes]

While running coversions from MARC to XML and then XML to RDF, there is some logic to consider when replacing an existing conversion file. Essentially, a "replace everything" flag can be set and this means everything will be converted again. When that is not set, the conversion process can check whether an output file exists or, when it does exist, whether the input file has been updated since the last time it was converted.

• [ ] MARC to XML
  • java code must check for input flags and file stats\
  • see http://stackoverflow.com/questions/32586/how-to-discover-a-files-creation-time-with-java
• [x] XML to RDF
  • shell script must check for env flags and file stats
  • this part is fixed by #24

[dazza-codes]

This issue requires some changes to MarcToXML. @jgreben is currently doing work on that to improve the unit tests, so I think this issue should wait on that, i.e. https://github.com/sul-dlss/ld4p-tracer-bullets/pull/88