Adds an output argument to EN_addnode and EN_addlink that returns the index of the newly added object.
Also refactors the validity check on object ID names.
The newly added `gettitle` and `settitle` functions were moved from the Reporting Functions section to the Project (formerly System) Functions section of epanet2.c and epanet.c.
In addition to addressing compiler warnings, argument names in the API function prototypes were made more consistent and descriptive. Also additional Doxygen comments were added in preparation for producing a more thorough documentation of the API.