Fix missing pressure plot at start of the dive in some situations

In some situations we could end up with no sample pressure and no
interpolated pressure at time = 0. This is now fixed.

Fix notes in test dive the exposed the issue.

Also change the code in create_plot_info to keep the number of samples and
the number of corresponding pi entries in separate variables. This avoids
future changes from breaking if they assume they can access
dive->sample[nr_samples - 1] (which is a reasonable assumption to make).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>