Interpolate depth for samples that have no depth

When downloading from libdivecomputer, we used to initialize the depth
of a sample to the previous depth.  However, at least for the Suunto EON
Steel, you can get sample times without any actual depth reading - the
time might be associated with some ranbdom event rather than a new depth
sample.

Rather than initialize these samples to have the same depth as the
previous one (and then perhaps getting a very sudden jump when the
*real* depth event comes in a second later), initialize the depth
samples to -1, and if that sample doesn't get a real depth, we'll create
an interpolated depth.

It is possible that we should just carry the sample around as not
actually having a depth, and instead just interpolate in the plot_info
generation, but at least right now we have a ton of code that "knows"
that every sample has a depth.  Not the least of which is our own save
format.

So generating an interpolated depth seems the path of least resistance,
and at least makes the graph look correct - no odd staircase effect from
other events that happen in between depth samples.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

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