Function to combine pre-saved MAGICC output with pre-saved MESMER local trend parameters and global + local variability emulations

[leabeusch]

Would be nice to have a function for this as this is the way I personally combine MAGICC and MESMER most frequently. Meaning, I take the probabilistic set of MAGICC and I combine each global trend trajectory with the ESM-specific local trend parameters and all the ESM-specific local variability realizations. If I also want global realizations with variability, each global trend trajectory of MAGICC can additionally be combined with all the ESM-specific global-variability emulations of MESMER.

One question to think about is if this function should just combine MAGICC's probabilistic set with the local paramters / variability realizations of a single ESM or directly of all ESMs MESMER calibrations are available for (& thus of all ESMs whose regional climate response uncertainty we can cover). Thus far, I've always used a brute force approach by combining each set of spatially resolved ESM paramters + variability realizations with each one of MAGICC's global trend trajectories and then stored the quantiles + some example realizations of the resulting ensemble... But of course this temporarily creates quite huge ensembles...