Merge pull request #335 from uwescience/store-temp-append

raco: support an AppendTemp operator
myrialang: fix appendtemp rule in case of double-scan

MyriaX cannot append to and read from a temp relation at the same time.
optimizer: test append_temp successful and failed

Test when both scanning from the left child and right child of the
union all.

Plumb kwargs in a few more places.