add Rathelot's index using mixture Betas approach for small-units

[renanxcortes]

One nice index to implement in the module is the one proposed in: https://amstat.tandfonline.com/doi/full/10.1080/07350015.2012.707586

Some previous work has already been done in that sense as Rathelot kindly shared with me original R code of this index and I'll put here more details in the future.

[knaaptime]

+1

[renanxcortes]

Some material regarding this issue can be found attached here.

Rathelots_Thread_Issue.zip