Added NEXAFS transitions calculation script.

transitions.sh uses the method of Teruyasu Mizoguchi, Isao Tanaka,
Shang-Peng Gao, and Chris J Pickard (J Phys: Condens Mat 21(10) 2009)
to calculate NEXAFS transition energies using the total energies
of a series of core hole calculations in comparison to a neutral
calcalation. Relies on calculate_transitions.py.
calculate_transitions.py supports transitions.sh

The python script does the maths for the method, that's
all. I just couldn't be bothered doing the maths in BASH
because the punctuation gives me a headache. :o)