Updates to DASSH access points and execution

Default DASSH execution is still as an installed package within Python, but if the user has specified in their .pyarc.rc a path to the DASSH executable shell script, it will run that way instead. The purpose of these changes is to streamline DASSH execution on shared computing resources (e.g. kookie).

In my .pyarc.rc file on kookie, I now have the following:

dassh_path = /software/dassh/

That's the directory in which I've put a file dassh.sh, which sets up a virtual environment (that I control) to run DASSH. I've just tested it on kookie and it runs well. @nstauff and @pshriwise - Perhaps we should have a meeting to make sure the changes I've made make sense?

(Additionally: various updates/bug fixes to PyDASSH and tests)