mergeExperiments: Merge data for two separate experiments
Description
The most common reason to merge experiments is across time periods--e.g.,
merging 'historical' with one of the RCPs. This function does that, checking
that the merge is appropriate and possible.
The variable, units, spatial grid,
depths/levels, domain, and model must all match. The timesteps must be
identical, and time values non-overlapping. If the time gap between the two experiments
is different than their internal timesteps (e.g., if two monthly data objects are
separated by more than a month) a warning will be printed.