sjmisc: Data and Variable Transformation Functions

New functions round_num() to round only numeric values in a data frame. General Improved performance for merge_df(). Furthermore, add_rows() was added as alias for merge_df(). merge_df() resp. add_rows() now create a unique id-name instead of dropping the ID-variable, in case id has the same name of any existing variables in the provided data frames. Improved performance for descr() and minor changes to the output. Support for mids-objects (package mice) Following functions now also work on mids-objects, as returned by the mice()-function: row_count(), row_sums(), row_means(), rec(), dicho(), center(), std(), recode_to() and to_long(). Changes to functions The weight.by-argument in frq() now should be a variable name from a variable in x, and no longer a separate vector. Bug fixes descr() does not work with character vectors, so these are being removed now