Develop a function fit_beta_mixture

[danielinteractive]

-> see daniel branch for design and first version (not reviewed)

To do:

• Design for a fit_mle(Beta, y, w) method such that the classic EM algorithm works, see Fitting a beta mixture fails with no method found dmetivie/ExpectationMaximization.jl#9 (comment) -> see design/fit_beta_mle.jl in daniel branch
• Add the fit_beta_mixture() function
• Add documentation
• Add tests

Note: the algorithm is not robust at the moment, probably mostly because of the Stochastic EM algorithm. Therefore might need to restart with another seed when resulting density is not approximating histogram well.