Access key. If not missing, it must be a
single character string that is partially matched against
the available algorithms in the registry. In this case,
if all=FALSE (default), then the algorithm is
returned as an NMFStrategy object that can be
directly passed to nmf. An error is thrown
if no matching algorithm is found.
If missing or NULL, then access keys of algorithms
-- that match the criteria version, are returned.
This argument is assumed to be regular expression if
all=TRUE or version is not NULL.