Specify whether to read shapefile `extr_proj` in procedure `shpc_open`
instead of just trying to read this shapefile. Use `uniform_lon_lat`
to decide whether to read the shapefile. As `shpc_open` is called in
both programs `inst_eddies` and `eddy_graph`, we cannot access
variable `uniform_lon_lat` of module `input_ssh_m`, which is not used
in program `eddy_graph`, nor variable `uniform_lon_lat` of module
`read_grid_m`, which is not used in program `inst_eddies`. So we add a
dummy argument `with_proj` to procedure `shpc_open`.