bibdocfile: possibility to enforce creation time
- Extends bibdocfile API to allow for optionally setting a timestamp that is going to be used as creation date insted of the running time of the API.
- Adds support for the modification timestamp in the FFT special tag via the $s subfield. (closes #1113)
Co-authored-by: Tibor Simko <tibor.simko@cern.ch>