Parse observation types to simulation code
parse_obs_types(obs, initial = FALSE)
obs: specified observation object including at least a description of which variable(s) are associated with a particular compartment, e.g. list(variable="CONC", scale="1").initial: is this for the initial code block in the C++ template that initializes the variables and compartments (TRUE), or for the second code block used for the rest of the dataset?