QM fix: Only use one rotational constant for linear molecules.

The rmgpy.statmech.rotation.LinearRotor.rotationalConstant expects only a single value.
But MOPAC or similar (via cclib) returns a moments of inertial result in an array, like 
   B = ([0, 68.4, 68.4], "cm-1").

I assume (as had @keceli in his version of this fix 210ea35697c725594e96a9c56e019aa61a8256b8)
that we want to use the highest of the frequencies (as opposed to the sum or anything fancy?)

If this is right, it closes #250