spew_mc: Run SPEW in Parallel with a Multicore backend
In spew: SPEW Framework for Generating Synthetic Ecosystems

Internal function, only called by the main spew function

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spew_mc(num_places, pop_table, shapefile, pums_h, pums_p, schools, workplaces,
  marginals, output_type, output_dir, convert_count, sampling_method,
  locations_method, outfile_loc, export_objects, road_noise, timer, verbose)

`num_places`	The number of regions (place_ids) to be generated for a given location
`pop_table`	dataframe where rows correspond to places where populations should be generated. Other requird columns are "n_house" and "puma_id"
`shapefile`	sp class object used for assigning households to particular locations
`pums_h`	dataframe with microdata corresponding to housegolds
`pums_p`	dataframe with microdata corresponding to people
`schools`	list with names "public" and "private" with a dataframe of schools corresponding to public or private, respectively
`workplaces`	dataframe of workplaces with a workplace_id column, employees column, and stcotr column
`marginals`	list of marginal population totals. Each element of the list contains the marginal totals of a separate variable
`output_type`	Default is "console" if we want to resulting population as an R variable. Alternative is "write", which is used on Olympus for writing out .csv files of the population
`output_dir`	directory to write output if output_type = "write", NULL otherwise
`convert_count`	logical meant to indicate if we are going to convert population totals from people to household counts. Default: FALSE, assumes the population is the total number of households
`sampling_method`	character indicating the type of sampling method to use, defaults to "uniform". Can also be "ipf" with appropriate marginal data.
`locations_method`	character indicating the type of location sampling to use, defaults to "uniform", can also be "roads".
`outfile_loc`	Defaults to "", so we print out the parallel run information. Only set to "/dev/null" for internal testing putposes.
`export_objects`	objects for exporting to parallel backends
`road_noise`	Noise added to households during road-based sampling
`timer`	logical indicating we want to time the run
`verbose`	logical indicating we want to print output during the run. Default is FALSE.#'