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>

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