Constructor for new rset objects
new_rset(splits, ids, attrib = NULL, subclass = character())A list column of rsplits or a tibble with a single column
called "splits" with a list column of rsplits.
A character vector or a tibble with one or more columns that begin with "id".
An optional named list of attributes to add to the object.
A character vector of subclasses to add.
An rset object.
Once the new rset is constructed, an additional attribute called
"fingerprint" is added that is a hash of the rset. This can be used to
make sure other objects have the exact same resamples.