fix null-pointer exception bug in IsomorphismHandlerCSP.java

[hoechp]

fix null-pointer exception bug in IsomorphismHandlerCSP.java!

...and test it thoroughly in general

(see #23)

[hoechp]

Found multiple bugs, already fixed some. Negative nodes in patterns are still not handled correcty, as it seemed. I presumed negative nodes would all have to be found for a pattern to fail, it is however now obvious to me, that even a single negative node, that matches to the otherwise 'found' match, already makes the whole match fail.

There are a whole bunch of other issues related to these things, though. Trying to work it all out now...

[hoechp]

what if a single negative node can be included to the otherwise 'found' match?

• the whole match will be disregarded

but what if a negative node has edges to another negative node?

• we'll have to find matches for both nodes, so that the match will fail. but we don't care in this situation if other, unconnected, negative nodes have matches or not

what does that all mean?

• all negative nodes, that are directly connected to each other end up in a seperate set of nodes to check for. if this set can be matched, the otherwise 'found' match is disregarded. if not, all the other sets are being checked one after another.

example: there is a pattern with 4 negative nodes (a, b, c and d). node a and b have some edge between them. so we check for each match that's otherwise found:

• does this match can be extended to match BOTH a and b with their connections between them? if so, the match fails
• does this match can be extended to match c? if so, the match fails
• does this match can be extended to match d? if so, the match fails