Avoid pointless calls to dive planner

Stupid Gtk. Seriously. So in order to get a notification if the user
selects the drop down for the gas with the mouse I need to connect to the
"changed" signal for the combobox. But that also fires whenever the user
types something into the GtkEntry. Which means we once again get called
for all kinds of silly partial names.

Instead we want to handle the manual entry in the "focus-out" callback
(the user has hit tab or something else to move away from the GtkEntry -
let's assume that this is the text he wants us to use) and only respond to
the changed signal on the combobox if the user selected something from the
dropdown.

The easiest way to do that (I think) is to check the text with the strings
stored in the model. If this indeed matches a string stored in the model
then most likely this is something the user selected from the dropdown.
But more importantly if it isn't in the model, then we KNOW that this is
just a partial string that was typed in. And we can ignore that one.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix gas handling in the planner

The dive will start with gas 0. If things change during the planned part
of the dive, this will be represented by an event. Use the last gas for
the ascent.

Obviously this still doesn't handle deco gases, but at least we now no
longer switch back to the first gas after the planned part of the dive.

This also adds quite a bit of debugging code to be able to trace what's
happening in the planner.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Rewrite of the deco code

o) Instead of using gradient factors as means of comparison, I now use
   pressure (as in: maximal ambient pressure).

o) tissue_tolerance_calc() now computes the maximal ambient pressure now
   respecting gradient factors. For this, it needs to know about the
   surface pressure (as refernce for GF_high), thus gets *dive as an
   argument. It is called from add_segment() which this also needs *dive
   as an additional argument.

o) This implies deco_allowed_depth is now mainly a ambient-pressure to
   depth conversion with decorations to avoid negative depth (i.e. no deco
   obliation), implementation of quantization (!smooth => multiples of 3m)
   and explicit setting of last deco depth (e.g. 6m for O2 deco).

o) gf_low_pressure_this_dive (slight change of name), the max depth in
   pressure units is updated in add_segment. I set the minimal value in
   buehlmann_config to the equivalent of 20m as otherwise good values of
   GF_low add a lot of deco to shallow dives which do not need deep stops
   in the first place.

o) The bogus loop is gone as well as actual_gradient_limit() and
   gradient_factor_calculation() and large parts of deco_allowed_depth()
   although I did not delete the code but put it in comments.

o) The meat is in the formula in lines 147-154 of deco.c. Here is the
   rationale:

   Without gradient factors, the M-value (i.e the maximal tissue pressure)
   at a given depth is given by ambient_pressure / buehlmann_b + a.

   According to "Clearing Up The Confusion About "Deep Stops" by Erik C.
   Baker (as found via google) the effect of the gradient factors is no
   replace this by a reduced affine relation (i.e. another line) such that
   at the surface the difference between M-value and ambient pressure is
   reduced by a factor GF_high and at the maximal depth by a factor
   GF_low.

   That is, we are looking for parameters alpha and beta such that

   alpha surface + beta = surface + gf_high * (surface/b + a - surface)

   and

   alpha max_p + beta = max_p + gf_low * (max_p/b + a - max_p)

   This can be solved for alpha and beta and then inverted to obtain the
   max ambient pressure given tissue loadings. The result is the above
   mentioned formula.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>