clim attributes and time model

[gleckler1]

@lee1043 pcmdi_compute_climatologies is working and still being tested for use in obs4MIPs. But two issues have come up:

1. the time model of the climatology needs to be improved. We need to follow the CF conventions
   http://cfconventions.org/cf-conventions/cf-conventions.html#climatological-statistics
   modifying the example "Example 7.9. Climatological seasons" for the case of "Climatological months"

2. enable global attributes to be removed/added/modified. It may be that the best way to do this is to operate on the climatology after it is written to a netCDF file, but much better if somehow it can be done on the fly.