Allow multiple pkl, ioe files to be passed to find_switch_events.py

[ajw2329]

This facilitates situations in which transcripts consistent with event orthologs might have different properties in different species. For instance, an "always" nmd event whose included form transcripts in one species all have PTC may be consistent with one additional included form transcript in another species that (putatively) lacks a PTC. Passing cds_insertion output and ioe files for both species allows the possibility of getting to a "consensus" result.