getMolecule now respects the charge parameter z

[janlisec]

This will fix Issues #4 and #7. It potentially breaks old workflows because people might have implemented some post processing on Rdisop results to fix the issue.

From the changeLog:
* exact masses will be corrected by z times the electron mass
* isotope masses will be additionally devided in case of abs(z)>=2
* a testthat function for getMolecule was set up
* the documentation for getMolecule was updated
* previous users will be informed by a message when calling getMolecule with z!=0

[sneumann]

Hi, if this is a breaking change, what about bumping the version to 1.99 ?
Yours, Steffen

[janlisec]

Your decision. :) It depends on how you see it. I believe as the parameter was doing nothing (except setting the charge slot), people did not bother to specify it in workflows. So, while it is leading to different results than before, the chance that it really breaks something is very low. It is rather a fix for #4 and #7.

[sneumann]

Convinced :-)