This is an accessor and replacement method for the
aqp_df_class entry in the metadata slot. This entry is used internally by methods that interact with
data.frame objects and slots to ensure that the same class used to promote to the SoilProfileCollection initially is used throughout the process.
# S4 method for SoilProfileCollection aqp_df_class(object) # S4 method for SoilProfileCollection aqp_df_class(object) <- value
"data.frame", "data.table" or "tbl_df"