replace #define statments for options and variable ids with enums, fixes 36
add comment clarifying the importance of leaving counter enum variables at the end of the list
add one last comment emphasizing the importance of leaving N_OUTVAR_TYPES and N_FORCING_TYPES as the last values of the enums - they are used as loop counters and must be larger than the largest member of their enum
Merge pull request #175 from UW-Hydro/feature/enums

replace #define statments for options and variable ids with enums