Add radius estimation method based on 2P King profile

[Gabriel-p]

Add 03/03/22 The r_lim estimation below (using the 3\sigma_b) makes little sense unless the \sigma_b is very small.

A method to estimate the r_lim using the 2-p King equation:

1. Fit the 2-parameter King profile (r_t=inf) using the RDP. This estimates te rc but also the f_0
2. Integrate the result, subtracting the integral of the field density
3. Select the radius associated to ~95% of the area

The f_0 value can be used in the estimation of the r_t later on.

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There's also an equation to estimate a r_lim value that I found in Bisht et al. (2020).

(f_bg is f_b but the f_b to the right is f(r))

This article says that this definition is taken from Bukowiecki et al. (2011), but it is not explained where it comes from. This last article mentions Peterson and King (1975) where the definition if the limiting radius is given as:

The limiting radius can be plausibly regarded as a real physical limit, beyond which the cluster cannot hold stars against the tidal force of the Milky Way. Again we define limiting radius operationally by means of the curves given in Paper III

The r_lim parameter can be obtained equating f(r) = f_b + 3*\sigma_b and solving for r.

Paper III (King 1966) and equates this "limiting radius" with the usual tidal radius.

In Maurya & Gour (2020) the r_lim is also used. It is obtained simply by assigning a density value at the position where the radius should be, and solving the equation. Thus the r_lim is an approximation to the tidal radius. The position where the radius "should be" is taken to be f_b+\sigma_b: