Refactoring: move methods from DataFile to Timestamp
Add support to use last modified information from file to enrich the date information of the time stamp (not the time values!)
Fix bug in lastModifiedDelta handling
Merge pull request #15 from EHJ-52n/feature/get-timestamp-from-lastModified

Feature/get timestamp from last modified

With this feature, it is possible to use the java.util.File.lastModified() information to enrich the date information of observations (not the time information) from a data file. In addition, it is possible to define a delta in days.