Consider outputting certain "blank nodes"

For a logical definition like this:

```
HP:Astrocytosis EquivalentTo RO:has_part some (PATO:increased_rate and (RO:inheres_in_part_of some (GO:cell_growth and (RO:occurs_in some CL:astrocyte))) and (RO:has_modifier some PATO:abnormal)
```

The term `HP:Astrocytosis` will have outgoing edges only where they are inferred to have named class targets, for example `RO:has_part PATO:increased_rate`, and any property chain inferences that result from the other nested targets.

Creating nodes for all the sub-expressions would allow more complete query, such as:

```
?s has_part:/inheres_in_part_of: cell_growth: .
```

The issue names no files, tests, or entry points, so first trace how logical definitions and nested existential expressions are materialized in the Scala project. Define completion as creating nodes for the relevant sub-expressions so queries such as `?s has_part:/inheres_in_part_of: cell_growth: .` can traverse the inferred structure.

Written by the indexing model from the issue text.