Additions to the EKP struct should be considered breaking changes

I am excited by the new functionality that #482 offers - it should streamline simulation evaluation. However, by adding to the EKP structure [here](https://github.com/CliMA/EnsembleKalmanProcesses.jl/blob/d00c76466dae087fced2c3776035088088766ca4/src/EnsembleKalmanProcess.jl#L218), it is "breaking" in the sense that I cannot read any EKP from an experiment run with an older version of EKP, even v2.4.0. As I've run several simulations with v2.4.0 that are rather expensive evaluation will be harder. I was wondering if:
1. We actually need to store `em` in the EKP object as opposed to just returning a dict when called.
2. We can bump minor versions as opposed to patches on breaking changes. Does CliMA have a policy for where breaking changes should be introduced? @charleskawczynski
3. We can fix this general problem where we can't read previously run experiments either with additional functionality (e.g. something like a `read_backwards_version` function) or changing how the EKP object is loaded. Currently JLD2 is not robust to these changes and perhaps we could load this with a failsafe method. (Or perhaps this exists already and I've missed it)