Speed up subclass checks

[fcbr]

Checking whether Animal is subclass of OrganicObject takes several minutes. I haven't been able to check whether Animal is subclass of Entity at all.

Ways of improving this:

• Follow sigma and cache the subclass hierarchy. Ideally all transitive relations could be cached, but subclass is used extensively;
• Investigate if something in our transformation is making this harder than it should be;
• Investigate if SUMO itself is making it harder than it should be;

[arademaker]

What could make things harder? Do you have any particular suspicion ?

[fcbr]

I tried a simple example involving completely artificial classes class1, class2, and class3 (to exclude problems that may be caused by partition, etc.) and it still takes several minutes.