hsp arguments in procedure `read_snapshot` can be declared with
`intent(in)` since they are `intent(in)` in `read_eddy` since commit d815bf76.

Bug fixes in procedure `get_snapshot`: remove duplicated declaration
of nlon, nlat; add argument `dist_lim`, which must be passed to `read_snapshot`.

In procedure `get_snapshot`, read the triplet of shapefiles from a
directory named with the date index (for the now moment, we will see
later for something more efficient). Check that the date index in the
shapefiles is the date index required.

In procedure `get_var`, take advantage of new procedure `nf95_get_missing`.