Add support for merging multiple simulations

[tangq]

During model development, sometimes it needs to change the simulation settings or code in the middle of a long-term run, but we want to analyze the time series of all simulations combined together. These simulations may have different names and noncontinuous years. It is not easy to apply MPAS-Analysis in this case.

An example can be found on the following two pages:
https://acme-climate.atlassian.net/wiki/spaces/EWCG/pages/2848456731/20210603.v2rc3c.piControl.northamericax4v1pg2+WC14to60E2r3.chrysalis
and
https://acme-climate.atlassian.net/wiki/spaces/EWCG/pages/3057877304/20210730.v2rc3c.piControl.northamericax4v1pg2+WC14to60E2r3.chrysalis