[ctioga2] make an explicit error on misspelled column names in the text spec
[ctioga2] test for error bars (and for ctable)
[ctioga2] allow a yerr= spec to be before a z column

... or more precisely, do not require the ?err columns to be at the end of the specs
[ctioga2] a bit more tests
[ctioga2] documentation and changelog updates