No need to load all assemblers

[lskatz]

Sorry if this is a nonissue but I think that Shovill requires all assemblers (skesa, spades, megahit) to be loaded in check_deps. If all dependencies are not loaded, then it calls err and exits with an exit code of 1.

Is there a way to remove the dependency check for assemblers that are not used? Is that something I could make a pull request for?

[tseemann]

It's to ensure people have installed their conda or brew package properly, and discourage people manually installing it.

Of course it doesn't need to be that way. I don't have a good track record of accepting PRs unfortunately, I get scared.