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

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>