Work with date index instead of date

This does not make the code simpler but a little more general,
anticipating for example time steps different than one day.

In main program unit `eddy_graph` and procedures `dispatch_snapshot`
and `read_snapshot`, variable k now refers to the date index, 0-based,
instead of the date in days since 1950-1-1.

In main program unit `eddy_graph`, add variable d for date. `k_begin`,
`k_end` and `k_end_main_loop` also now refer to date indices. We have
to pass an additional argument `d_init` to procedure
`dispatch_snapshot`.

Note that no modification is necessary in procedures `get_snapshot`,
`recv_snapshot` and `send_snapshot` although the meaning of k in
`get_snapshot` and tag in `recv_snapshot` and `send_snapshot` also
changes.