The valid_data_module function takes a path to a TDFF data module as
input parameter and checks its validity. In detail, the function checks
the data folder of each of the data module versions in path for
required data files and content.
valid_data_module(path)character(1) with the (full) path of the data module. The
specified directory should contain sub-folders with different versions
of the data module.
The function returns (invisibly) TRUE if the data module is valid,
or throws an error.