The Influence of Sulphur on Anharmonic Features of Vibrational Circular Dichroism, on Electronic Circular Dichroism and on Optical Rotatory Dispersion: A Study of (R)-2-Chloromethyl-Oxirane, (R)-2-Methyl-Oxirane, (R)-2-Chloromethyl-Thiirane, and (R)-2-Methyl-Thiirane