Make CsvCompound's child_key setting optional

[mjordan]

Currently the CsvCompound toolchain's [FILE_GETTER] child_key setting is required in the .ini, but if the CSV metadata file does not contain any rows for child metadata, each row must have a column matching the field identified in [FILE_GETTER] child_key and it must be blank. This is a lot of overhead in preparing the CSV and the .ini file.