Create spectroscopy-letters

I've started working from the applied spectroscopy style as mentioned in the summary. 
There are still a few issues, but at least it seems to be compliant to 100% with journal references
Issue: a comma after the date still appears in the book references and I didn't know how to remove it. Probably it works with conditionals...)
Yes that is the direct link to the instruction page, but the validation programm identified an error with this address, so I thought it would be safer to give the link of the general journal webpage instead of the direct link to the instructions