As simpler `run_tardis`

Using the `run_tardis` function has been a bit annoying (one needed to first open the yml file and then give it to run_tardis). This function update hopefully made it a lot simpler (@unoebauer see if this is to your liking)

@mklauser @ssim @orbitfold one of you please sign off (or object) so we can merge this. 
@mklauser You were interested in duck-typing. This is a classic example. You could do a type check to see if is as string, but one simply opts to give it to the python `open` command and then sees if it works. This means also unicode strings can work, or other things that open can read.