[DOC] twocpt_population is not a hierarchical population model

[billgillespie]

Description

The twocpt_population model in the example-models directory is not an appropriately constructed population model, i.e., a hierarchical model with inter-individual variability in model parameters. It is instead a simultaneous fit of a set of individual models---one model per individual. While this is suitable for demonstrating the use of the pmx_solve_group functions, it does not demonstrate good PK modeling practice. The model should be replaced by a correctly structured model.

Expected documentation

Revise or replace twocpt_population with a corrected model. This also needs to be changed in the Users Guide.

Current Version:

v0.90.0

[yizhang-yiz]

I can do this during the holiday break.