ISSUE-176: Rules will not modify the data graph

 From the recent meeting minutes I can see that Ted remarked [1]

long ago we decided that SHACL engines would be fed a graph which it 
would validate, and that SHACL engines would not change that graph 
before validation ... but this reverses that and re-opens many past 
decisions

I agree with the previous decision and notice that the wording in the 
proposed section was not clear. I have changed the wiki page to clarify 
that the execution of rules happens *before* the data graph is produced, 
i.e. the data graph is the result of applying rules on some other 
"input" graph. Rules will not modify the data graph, but operate in the 
same way that other entailments are implemented.

Holger

[1] https://www.w3.org/2016/08/25-shapes-minutes.html#item04