Code Cleanup/Refactoring

[andersy005]

• Removes time_manager class and replaces it with an esmlab accessor on xarray datasets
• Refactors utility modules: most utility functions are now in core.py and accessible via esmlab accessor.
• Fixes some bugs in compute_ann_mean
• Removes climatology.py module and moves climatology functions to core.py
• Regridding utilities are now implemented in Code Refactoring of existing functionality esmlab-regrid#3
• Drop Python 2.7

[andersy005]

@kmpaul, do you mind reviewing this whenever you are not swamped with work and/or other responsibilities :)? Thanks!