Will usearch64 work the way it's entered?

[mhoban]

Right now the usearch process runs in the context of the usearch(32) singularity container, so it may not actually work to just point it to the usearch64 executable.

To be safe, let's make a third process called something like usearch64_relabel/derep that runs without a container so it runs the executable in the system context.

[mhoban]

Also the code may be all wrong if the user supplies an executable usearch.

[mhoban]

this is probably a duplicate of #47, more or less