added provenance support, preliminary tests pass
added prov test cases
fixed bug in flatten.  supports tables with and without table columns
added more exprs to filter desc.
refactored stealcols to a variable
added @roots method to traverse
set row ids in all/each methods.   add roots method to table
ensure(): canonical row steals values of shared columns from left table
HACK: add svg attribute to track geom elements in row
update gitignore
added unique row ids for every operator
added parttioned pairtable support
added parttioned pairtable support
fixed provenance bugs in partitionedpairtable so dependence on left() is propogated correctly