Add a reader for cdxml format

[hhaensel]

added a reader for cdxml format that supports

• bond stereo information via 'notation' analog to sdfilereader
• aromaticity from bond order '1.5'

[hhaensel]

@mojaie would be great if you could review

[hhaensel]

@mojaie sorry for pinging you once more on this. I had the impression that you liked the idea of adding a cdxml reader?
If it's just lack of time, you might leave a short note.

[mojaie]

I'm sorry for the late response.
It looks good to me. If there is any specific reason why you chose LightXML.jl instead of EzXML.jl, please let me know.
I’m ready to merge whenever you are.

[hhaensel]

LightXML was a pick from the AI, but as EzXML is meanwhile hosted by JuliaIO, it's probably the better choice, moreover we're using it in GenieFramework.
So I took some time to switch to EzXML and cleaned up the code a bit.
The only two functions to be exported are cdxmltomol and cdxmltomols. They have similar syntax as sdftomol. Morevover, there are CDXMLAtom, CDXMLBond and CDXMLMolGraph.
So I guess it's ready for merging.

[mojaie]

Thank you very much!