Avoid overwriting external clustering in visualise script

[nickjcroucher]

@EmmaLRussell pointed out beebop is being scuppered by the overwriting of external clustering in the visualisation code - I think this fixes the problem and at least adds a test for running this mode of visualisation, although we might need to check it actually outputs the correct clustering at some point. But if we can release to conda we can get the new version of beebop out soon.

[johnlees]

Looks good, I will make a release now so we can get on bioconda tomorrow