ignore (some) empty field when converting csv

This fixes bug #733 and bug #734.
update baseline test for convert command
ignore extra fields when converting csv files

This fixes bug #736.
Merge pull request #89 from kljohann/bugs.convert

fix several ledger convert bugs
Merge branch 'master' into next

Some commits were on the master branch but not yet on the next branch due to a
pull request to master after those two branches diverged. This merge
ensures that next ⊇ master.