'Str()' is an enhanced 'str()'. If the object is a data frame with atomic columns,
this function captures output of internal 'str()', changes it and outputs the new one.
If the object is not a data frame or is a data frame with non-atomic columns,
then output is not changed.
'Str()' (1) shows data frame structure with column indexes,
(2) indicates presence of NA(s) with star (*) and
(3) lists first five row names, if they are not default.
'Str()' does not work (therefore, passes everything back to common 'str()')
with data frames which have non-atomic columns (fortunately, rare case).
Alternative: DescTools::Str() which uses cycles (slower!), has less features, but works with non-atomic columns.