Move initialization of shapefiles to a separate procedure,

init_shapefiles. This allows us to have variables for DBF field
numbers, as module variables. We avoid the burden of passing this
large number of variables as arguments to write_eddy. Safer to have
variables for DBF field numbers than relying on the order of those
fields.

Remove file_list. This was a duplication of information.

Use named constant for missing max speed value. It is used in two procedures.