Update metaclusterers.py

[AvantiShri]

Attempt to patch the scalability of the metaclustering code

[AvantiShri]

@tchiruvolu I haven't tested out this code, but can you try out this fix for the metaclustering scalability? To install this branch of tf-modisco you need to do:

pip uninstall modisco #uninstall your current version
git clone https://github.com/kundajelab/tfmodisco.git
cd tfmodisco
git checkout AvantiShri-metaclusteringpatch
pip install --editable . #the --editable flag just means that if the soruce is edited later, the changes will get picked up

I basically changed two things as you can see in https://github.com/kundajelab/tfmodisco/pull/42/files; (1) all_possible_activity_patterns is now only populated according to activity patterns that appear in the set of seqlets, and (2) the set of all possible compatible activity patterns is generated in a runtime proportional to 2^number_of_tasks rather than 3^number_of_tasks.