FlowParser: Allow referencing excluded rows/blocks

[geoo89]

Rows or entire blocks of rows may be excluded via the include_if column.
Currently, if a row_id of a row that is excluded is referenced in a go_to row or in a from column, and error is thrown.
We might want to implement excluded content in such a way that references to it simply yield the next included content.

[geoo89]

This can likely be implemented by implicitly using no_op node groups.