The file argument was renamed to out_file for consistency with the
other convert_*2xlum() functions (#1).
Improved input object validation (#3).
Objects with .uid slot set to NA and no parentID specified in
the info slot field can now be converted correctly (#5).
The attributes from the xlum_prototype file are read only once and
then cached, rather than at each function call (#7).
The objects generated by the function no longer produce validation
errors when checked with validate_xlum() (#11).
The generation of the startDate field for Daybreak files has been
made more robust to accommodate the various date formats used by this
file type (#15).
The conversion of RLum.Data.Spectrum objects has been fixed to set
the tValues, xValues, xLabel, xUnit, vLabel and vUnit
attributes correctly (#17).