Implement util function to sample model parameters from population

[DavAug]

• Create a Utilities module
• Move compute_pointwise_loglikelihood to Utilities module
• Implement a function called sample_population
• Takes posterior_samples in form a xarray.Dataset
• Takes list of erlo.PopulationModel
• Names of the modelled parameters
• Returns a xarray.Dataset with samples from the population model of the same size as samples in the posterior_samples.
• The individual coordinate is 'population'
• For pooled and heterogeneous models the samples can just be copied.