updating workplace generation so it matches new *location*_distribution_generation & *location*_distribution_country pattern

[cwhittaker1000]

As discussed in https://github.com/mrc-ide/helios/issues/83#issuecomment-2180619194 we don't need new data for USA workplaces (what we have currently from Ferguson et al 2006 is fine).

This PR updates get_parameters() and create_variables() to accommodate the new pattern we're going to use for generating location sizes in general (and matches that of https://github.com/mrc-ide/helios/pull/97).

This new pattern uses two arguments location_distribution_generation (which can take either "synthetic" or "empirical") & location_distribution_country (which for workplaces we only have "USA" currently).

[cwhittaker1000]

Get started running this with:

# Loading library
library(helios)

# Checking country is present as new argument and can be overridden 
model_params <- get_parameters(overrides = list(workplace_distribution_country = "UK"))
model_params$workplace_distribution_country
model_params <- get_parameters()
model_params$workplace_distribution_country

# Checking create variables works with USA and UK
uk_variables <- create_variables(get_parameters(overrides = list(workplace_distribution_country = "UK"))) ## returns error correctly as we don't have data for UK on this

usa_variables <- create_variables(get_parameters(overrides = list(workplace_distribution_country = "USA")))
usa_workplace_id <- usa_variables$variables_list$workplace$get_categories()
usa_workplace_sizes <- vector()
usa_workplaces <- 1:max(as.numeric(usa_variables$variables_list$workplace$get_categories()))
for(i in usa_workplaces) {
  usa_workplace_sizes[i] <- usa_variables$variables_list$workplace$get_size_of(as.character(i))
}
hist(usa_workplace_sizes, breaks = 250)

## Checking run_simulation works with it
usa_results <- run_simulation(get_parameters(overrides = list(workplace_distribution_country = "USA"), archetype = "sars_cov_2"))
plot(usa_results$timestep, usa_results$E_new, type = "l")

[cwhittaker1000]

Comments addressed, and confirm this works with:

# Loading library
library(helios)

# Checking country is present as new argument and can be overridden 
model_params <- get_parameters(overrides = list(workplace_distribution_country = "UK"))
model_params$workplace_distribution_country
model_params <- get_parameters()
model_params$workplace_distribution_country

# Checking create variables works with USA and UK
uk_variables <- create_variables(parameters_list = get_parameters(overrides = list(workplace_distribution_country = "USA"))) ## returns error correctly as we don't have data for UK on this

usa_variables <- create_variables(get_parameters(overrides = list(workplace_distribution_country = "USA")))
usa_workplace_id <- usa_variables$variables_list$workplace$get_categories()
usa_workplace_sizes <- vector()
usa_workplaces <- 1:max(as.numeric(usa_variables$variables_list$workplace$get_categories()))
for(i in usa_workplaces) {
  usa_workplace_sizes[i] <- usa_variables$variables_list$workplace$get_size_of(as.character(i))
}
barplot(table(usa_workplace_sizes), breaks = 250)

## Checking run_simulation works with it
usa_results <- run_simulation(get_parameters(overrides = list(workplace_distribution_country = "USA"), archetype = "sars_cov_2"))
plot(usa_results$timestep, usa_results$E_new, type = "l")

Have also updated everything to match the naming convention/approach we're taking where we just specify the country and ignore parameters passed in unless the country is set to custom.