add writeMapping, update tutorial, explain source column, convert to LF

[orichters]

Purpose of this PR

• using different line endings in the package leads to annoyingly large diffs between windows and linux users. Convert everything to LF and make sure with .gitattributes it stays like that
• improve the tutorial with respect to price indices and the source column, also improve testing on this on by adding a inst/sources.csv file with all options so that can be checked.
• move adding an interpolation column out of getMapping to make sure one can edit the files with R without adding this column. Instead, add an empty interpolation column if missing in generateIIASASubmission
• add writeMapping function with correct write.table settings. Add a test that writing and reading all mappings returns the same result.