Morphines split throws ConcurrentModificationException if inputField==outputField

If a morphlines split command has the same inputField and outputField, a ConcurrentModificationException is thrown.

Presumably this is because the field is still being iterated over when the new split values are written to the field.

I suspect (but have not tested) that the same issue would occur when multiple outputFields are used instead.

At a minimum, the documentation could warn that the input and output field names must be different.

Better, the command could check the input and output field names and emit a more meaningful exception.

Ideally, I would like to see this work without error, with the option to either add the split values to the existing values, or overwrite the existing values with the split values. I guess the latter should probably be the default.

Workaround: either a) change the outputField name, b) give the inputField a different name in the first place, or c) copy the input field to a temporary field and delete the existing values before splitting the temporary field back into the original field.