Make LaTeX Citation more robust

Hi,

this pull request should (hopefully) fix the following issues:

- c48a26a: #72 Bug in cite completion
- 1e33112: #74 Problem with cite when one of the Bib files is not found
- c48a26a: The Citation command tried to search file locations which are (usually) part of a generic macro. E.g.: We would try to locate the file `#1` and fail if we have the macro described below. In reality `#1` is only a placeholder for the first argument — the real file name — of the macro.
```latex
\newcommand{\codeinput}[1]
{
    \begin{leftbar}
        \input{Code/#1}
    \end{leftbar}
}
```
Kind regards,
  René