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...)
This is your comma you want to delete. But I guess you want to have a comma if either a locator is following or the access information?

If yes, then I would suggest to substitute below in the bibliography to something like this:
```
<group delimiter=", " suffix=".">
  <group delimiter=" " suffix=".">
    <text macro="author"/>
    <text macro="title"/>
    <text macro="editor"/>
    <text macro="container-title"/>
    <text macro="edition"/>
    <text macro="publisher"/>
  </group>
  <group delimiter=" ">
    <text macro="location"/>
    <text macro="access"/>
  </group>
</group>
```
(Note: not tested)