use versioneer for standard versioning

[gidden]

Previously, we tried to import the package to get the version number in setup.py. This broke our conda build. It appears that versioneer can fix this. See, e.g., xarray.

[gidden]

this is not possible given that we do versioning in the GAMS code. we may want to think about revisiting this strategy as there is now non-trivial amount of code (e.g., message_ix.Scenario class). @danielhuppmann let me know what you think. Closing this for now, but we may want to have two versions here, the code and the model.