Return `output` filepath `str` within pycytominer functions

[d33bs]

Currently output doesn't return anything when used through, for example, normalize or other functions. It'd be nice to be able to label the return or chain it directly in other functions when appropriate (e.g. annotate -annotated.parquet-> normalize -normalized.parquet-> etc.). This would follow the convention of output already providing a return str. It would also enable a kind of declarative syntax where we can make use of the return from the function as opposed to a variable filename (which is more loosely associated with the function's output).