Check if an object is of class das_df, or coerce it if possible.
as_das_df(x)# S3 method for das_df
as_das_df(x)
# S3 method for data.frame
as_das_df(x)
an object to be coerced to class das_df
An object of class `das_df`
Only data frames can be coerced to an object of class das_df.
If x does not have column names and classes as specified in das_df-class,
then the function returns an error message detailing the first column that does not
meet the requirements of a das_df object.