Write number of days since 1950-1-1 as date_index in shapefiles

(instead of date_index = 1). This is the value read from the NetCDF
files. We have to increase the width of field date_index to 5 digits
(4 digits would only allow approximately 30 years since 1950).

In procedure read_snapshot, check that all input eddies have the same
date index, and return it as an additional argument.

Assume that the input NetCDF files contain a single date.

In program test_set_all_outerm, do not write file
"isolated_nodes_1.csv". Since s is defined by set_all_outerm only, all
the eddies are necessarily isolated.