Check observations for reserved grouping variables
See also
Functions used for checking inputs
check_design_matrix_sparsity(),
check_group_date_unique(),
check_max_delay(),
check_module(),
check_modules_compatible(),
check_numeric_timestep(),
check_observation_indicator(),
check_quantiles(),
check_timestep(),
check_timestep_by_date(),
check_timestep_by_group()
