DTSg v0.6.0

• Added subset() method: allows for filtering rows and/or selecting columns of a DTSg object.
• Added setCols() method: allows for setting the values of columns of, adding columns to and/or removing columns from a DTSg object.
• Added [ extract operator: acts as a shortcut for the getCol() method.
• Added examples to the documentation of the colapply() method showing how to calculate moving averages with the help of the runner package instead of the rollapply() method.
• aggregate() method can benefit from data.table's GForce optimisation now when its fun argument is provided with a character vector specifying summary functions.
• Greatly sped up nas() method.
• Temporal aggregation level functions supplied to the funby argument of the colapply() method are not forced to return a POSIXct timestamp any longer. They are, however, forced to return an atomic mode (the same goes for the subset() method).
• getCol() method now is capable of also querying the .dateTime column.
• R6Method argument of S3WrapperGenerator() function now also takes a public method of an R6ClassGenerator as a function and not only as an expression.
• Fixed that not all missing values were made explicit after a call to the merge() method despite an "explicit" na.status in some cases.
• Fixed that getCol() method tried to query all numeric columns instead of only the first one by default.
• Improved vignettes and documentation.
• Minor internal code improvements.