opts_FRK is a list containing two functions, set and get, which can be used to set options and retrieve options, respectively. Currently FRK uses three options:
"progress":: a flag indicating whether progress bars should be displayed or not
"verbose":: a flag indicating whether certain progress messages should be shown or not. Currently this is the only option applicable to method = "TMB"
"parallel":: an integer indicating the number of cores to use. A number 0 or 1 indicates no parallelism