... (and other Exodus-session filetypes) when using the
"export smtk model" followed by "import smtk model" operators
in different sessions.

This commit also

+ moves common JSON export tasks out of the I/O delegate
  methods and into ImportJSON/ExportJSON.
+ Bumps the required Boost library version to 1.60.0
  in order to use relative path functions that are new.
+ Adds a test of import and export as the operators
  were not being tested.