Add cachetools to MOCK_MODULES list in doc build config.
Remove cachetools, add chash and xxhash dependencies, use chash method memoization decorator in pattern.py.
Explicitly specify hash function to be used in pattern.py.
xxhash -> xxh; no need for explicit link dependency now that xxh is on pypi.
xxhash -> xxh in docs conf mock list.
Merge branch 'port_ids' of https://github.com/neurokernel/neurokernel

Conflicts:
	neurokernel/pattern.py
Add additional index construction method to address issues defining patterns between selectors with different numbers of levels.
Fix handling of patterns with single-level selectors in Pattern.is_connected().
Merge branch 'port_ids' of https://github.com/neurokernel/neurokernel
Made interface faster for gpot data
Changed to two level selectors for retina_lamina example
Make get_intensities faster in eyeimpl.py
Change example in core.py to use selectors with multiple levels