[REFACTOR] Introduce high-level sampling methods and make specific methods internal

[hsloot]

Summary

The public interface should have only a few high-level for selected (sub-)classes: rmo (arbitrary MO distribution), rexmo (exchangeable MO distribution), rextmo (extendible MO distributions), and rpextmo (parametrized families of extendible MO distributions).

Proposal

The signatures should be as follows:

• rmo(n, d, intensities, method = c("AM", "ESM", ...)) , where n is the number of samples, d is the dimension, intensities are the shock-arrival intensities.
• rexmo(n, d, ex_intensities, method = c("MDCM", "AM", "ESM")), where ex_intensities are the scaled, exchangeable shock-arrival intensities, i.e., the exchangeable shock-size arrival intensities.
• rextmo(n, d, psi, a = 0, b = 0, scale = 1, cscale = 1, method = c("MDCM", "AM", "ESM")), where psi is a Bernstein function, a and b are coefficients for a (quasi) affine-linear add-on, scale is a the scale for the Bernstein function, and cscale is the composite-scaling of the Bernstein function.
• rpextmo(n, d, family, eta,...) is a drop-in replacement for rextmo(n, d, new(family, nu), a, b, scale, cscale, method); only if method == "LFM", the method will work differently.

All methods should check input parameters extensively, e.g., that new(family, nu) returns a valid Bernstein function or that the "LFM" method is applicable.

Checklist

• [x] Implement, test, and document rmo.
• [x] Implement, test, and document rexmo.
• [x] Implement, test, and document rextmo.
• [x] Implement, test, and document rpextmo.

[hsloot]

Should be done after #81.