Copy iterable selector passed to PathLikeSelector.expand() to prevent it from being modified by the method.
Add is_empty() method to PathLikeSelector, add tests for is_empty() and is_expandable().
Remove redundant is_empty() method, modify methods in PathLikeSelector to check whether non-str selectors are/contain lists/tuples.
Update are_disjoint() method to treat the empty selector as disjoint to all selectors.
Merge branch 'port_ids' of https://github.com/neurokernel/neurokernel
fixed bug in photoreceptor, added option for simulation of image with varying brightness