DTSg v0.5.0

• Added getCol() method: allows for querying the values of a single column of a DTSg object.
• Added funby and ignoreDST arguments to colapply() method: allows for applying functions like cumsum() to a certain temporal level.
• Added na.status argument to new() and alter() methods: allows for making missing values either "explicit" (default) or "implicit" or leaving them alone via "undecided".
• Added na.status field reflecting the status of missing values (can also be actively set).
• Added na.status also to the list of helper data passed on to temporal aggregation level functions.
• funby argument of aggregate() method now also accepts a named list of functions: allows for calculating several summary statistics at once.
• periodicity field can now be actively set in order to change the periodicity of the time series.
• timezone field can now be actively set in order to convert the time zone of the series.
• Fixed that a DTSg object with only one timestamp accepted a missing value in its .dateTime column.
• Improved vignettes and documentation.
• Minor internal code improvements.