Allow transportData to be given its epsilon in units of Kelvin.

It is stored internally as an energy, and so is converted from
K to kJ/mol by multiplying by R.

Perhaps this class should be cythonized (the LennardJones that it 
replaces was, and it may be used a lot in the PDep code, I'm not sure...)
Convert pdep/networkTest unit tests to use new TransportData for LennardJones

Reduces the number of failing unit tests.
The LennardJones class that we have replaced used to be Cythonized.
I wonder if it is used a lot in the pdep code, and we have made things
much slower...  I guess time will tell.