Pass variables to cluster while using use_cluster

[aatishk]

I am using look_up function to allocate values to a model parameter. The look_up function peeks into a data frame. This data frame is not passed to all the cores in the cluster when using the use_cluster, which causes use_cluster to fail.

parallel package (which is being used by heemod to parallelize) has the option to pass on variables using the clusterExport function. It'll be nice to have the option in heemod to pass variables/data-frames to all the nodes in the cluster while creating a cluster.