Implement process chain like "|cleanData()|accum()", performing each command of the sequence.
container for the logic for slicing at an index vs slicing at a datum. If the string is an integer, then we return the index. If the index is a string, then we need to find the corresponding index to the rank 0 dataset elsewhere. If the string argument is not parseable, then deft is returned.
process implements the poorly-named filters string / process string of Autoplot, allowing clients to "pipe" data through a chain of operations. For example, the filters string "|slice0(9)|histogram()" will slice on the ninth index and then take a histogram of that result. See http://www.papco.org/wiki/index.php/DataReductionSpecs (TODO: wiki page was lost, which could probably be recovered.) There's a big problem here: if the command is not recognized, then it is ignored. We should probably change this, but the change should be at a major version change in case it breaks things.