Only supports thermo_style one

[owainkenwayucl]

First of all this library is excellent and really useful - I'm working with an MSc student who is using it as part of an automated workflow.

One issue we found though is that if you set thermo_style multi in the input file (as for example in the Rhodospin benchmark that comes with LAMMPS) it can't read the output - setting it to thermo_style one of course fixes this!

A symptom of this is that the .get(...) methods return empty series for sensible names and if you get the library to print out the valid keys it's full of things like "-----" etc.