Combine two MultiSeries objects

[kcpevey]

We need a way to combine two MultiSeries objects. I'm opening this as a separate action item, but I actually think it can be handled in the same way as #18 - via a MultiSeries.add method.

MultiSeries.add(MultiSeries) could detect accept different types (Series, MultiSeries, list of Series) and handle them appropriately.

[CommonClimate]

yes, and same as #18: overloaded + or & method would be most valuable as a shorthand.

[CommonClimate]

the new Series.equals() method should help flag duplicates, and remove them if necessary

[MarcoGorelli]

from our side, is this one covered by #18, or is there anything else which needs doing here?

[CommonClimate]

it is not covered. See the latest version of Paleopandas Playground.ipynb for an illustration. AFAICT, #18 only allows to a a Series to a MultipleSeries object, not to combine two MultipleSeries objects. This still needs to be done by the end of the project, but I'm guessing won't be huge ordeal?

[CommonClimate]

Addressed by #354. Still needs a notebook tutorial, or at least informative docstrings on and and add to highlight usage differences.