xml parsing functions +

[andybeet]

Functions/files

• xml_convert.r file that contains functions to read, parse and convert Ecobase xml file into a valid rpath object
• documentation of functions with examples
• vignette explaining use
• Added "West Bering Sea.eiixml` as example to be used in vignette and examples

Documentation fixes

• fixed some broken links in function docs
• removed duplicate rpackage doc
• added files buildignore

[andybeet]

@kaydin Not sure if you also want to add a line in the ModelSetup.rmd to tell user they can also import their model from EwE instead of building from scratch in R.

Can you also check documentation in the 2 exported functions and flesh out the vignette?