The Data Processing Elements specifies the input elements and processing algorithms
to generate harmonized variables in the DataSchema formats. It is also
contains metadata used to generate documentation of the processing.
A Data Processing Elements object is a data frame with specific columns
used in data processing: dataschema_variable, input_dataset,
input_variables, Mlstr_harmo::rule_category and Mlstr_harmo::algorithm.
To initiate processing, the first entry must be the creation of a harmonized
primary identifier variable (e.g., participant unique ID).