initializer

Initializer in the Invoking Subclass

When we reach
```
    $dataref = $self->init(...);
```
... we are instructed to call the version of _init() found in the invoking subclass -- in this case Data::Presenter::SampleCensus::_init().
That _init() function parses data from a specific sourcefile.
We would write a different Data::Presenter subclass with its own _init() subroutine to parse a different sourcefile.
But in each case we would re-use the parent Data::Presenter module's constructor and output methods.