Purge op2 common

Remove dependencies to OP2-COMMON
Split the old op_lib_core extension into separate Extensions (plan, sparsity, compyteinds)
Integrates helper C++ code (sparsity_utils.cxx) directly in cython

buildbot is happy so far http://buildbot-ocean.ese.ic.ac.uk:8080/builders/pyop2-testing/builds/412