Add rcParams entry for bfmi threshold

[aloctavodia]

This has been proposed by @tomicapretto in arviz-devs/arviz-plots#390

People have been using 0.2 and 0.3. Relatively recently, on the Stan side, they unified their libraries to use 0.3.

[sanjiti-gill]

Hi, I’d like to take a look at this if it’s still relevant.
Since the issue description is brief, I wanted to check first, is the expectation simply to expose the BFMI threshold as a configurable rcParams entry following existing patterns, or are there additional considerations I should be aware of before starting?
Happy to proceed once I have a bit more context. Thanks!

[aloctavodia]

If we agree to have this, we need to add the entry and then make plot_energy get the value from rcParams.