Merged "read-xml-file" and "read-file" into "read-file"
* FunctionReadFile is now able to distinguish between XML files and plain text files
** it selects the appropriate course of action automatically
** reading the file as XML into the DOM or reading it as a string
* the current selection criteria is the file extension
** I am thinking about trying to import every file into the DOM and using the result state of that action as selection criteria
* Updated README.md and test cases accordingly