Return a list of all possible ids that can be used in the 'from' argument
Usage
cts_from(verbose = getOption("verbose"))
References
Wohlgemuth, G., P. K. Haldiya, E. Willighagen, T. Kind, and O. Fiehn 2010The Chemical Translation Service – a Web-Based Tool to Improve Standardization of Metabolomic Reports. Bioinformatics 26(20): 2647–2648.