[BUG] - Fix description of time constant estimation

[TomDonoghue]

We have a function the repo to estimate 'timescales', added in here. I happened to notice that the function we have here described the calculation as being estimated from the knee parameter, whereas Richard's work estimates it from the knee frequency - which I assume is correct? If so, this is currently incorrectly described here, and this PR updates that.

This is what I'm following from Richard's work:
https://github.com/rdgao/field-echos/blob/master/echo_utils.py#L65

I know the "knee frequency" calculation is an estimation - this function should at least be updated to describe the correct parameter - should we also add a bit of note describing it more and/or referencing this to Richards paper? Thoughts, @rdgao?

[rdgao]

yes that's exactly right - I noticed this at some point myself and forgot to do anything about it.

but yeah, in the function/equation in question it should be the knee_frequency, and that doesn't involve estimation, it just converts frequency to time and that's always right.

What might be a good thing to point out is in the function above (compute_knee_frequency, L8), that that involves a non-exact approximation depending on exponent. I didn't find any references that do this for the "general" Lorentzian case when I was working on it, so I'm still not sure if it's right. But maybe make this reference so people know who to blame.

[TomDonoghue]

@rdgao - sounds good, thanks for the info!

I added a new commit that adds explicit references to your paper, and also adds a note on the conversions, describing what you mentioned about it being an estimate rather than a precise quantity - do you want to take a quick look and double check it captures what you want it to?

[rdgao]

yeah looks good to me!