Standardized on D versus d_t for Dataflow Fact

[blipper]

Some places use D and some use d_t. We should just pick some to standardize on the template type name.

[MMory]

I agree on that. We intend to go with d_t in the future. This might be a low-hanging fruit for newbies :) I just labelled this as good first issue.

[fabianbs96]

Is this still an issue? We added an entry to our coding guidelines were this is specified.