Possible minor bug in determining location of runs in campaign when work_dir not set

[d7919]

With EasyVVUQ v1.1 installed through pip I found that when not specifying work_dir to Campaign that whilst my campaign database appeared in a sub-directory of the current working directory, the runs appeared under /tmp.

I think the issue is with the below line, which I think should use self.work_dir rather than work_dir?

EasyVVUQ/easyvvuq/campaign.py

Line 158 in fac0b57

self._campaign_dir = tempfile.mkdtemp(prefix=name, dir=work_dir)

[d7919]

Possibly also applies to https://github.com/UCL-CCS/EasyVVUQ/blob/dev/easyvvuq/campaign.py#L220

I'm just getting up and running with the code so may be misunderstanding the intent here, but the change given in PR #360 does seem to change runs from appearing under tmp to the expected location under the current directory in my setup.